At Studio 54 on the eve of the Canadian election, 1979

My day with Margaret Trudeau—the glamorous former first lady of Canada, 1970s tabloid fixture, and mother of the country's handsome, young, newly elected prime minister, Justin Trudeau—begins with an unscheduled 8 A.M. phone call to my hotel in Montreal. "Hello, it's Margaret Trudeau," she intones, the slightest hint of a Canadian accent in the way she casts her vowels. We were scheduled to meet later in the day, at 2 P.M., but Margaret asks me if I'd like to get together beforehand so we can get to know each other. She offers to pick me up herself. "I'll meet you outside your hotel," she says. "I'll be the one in the beat-up Prius."

A few hours later, a dented red sedan pulls up in front of the Hotel Le St-James in Old Montreal, the back piled high with sports equipment and children's car seats. "Hiii-iii," says Margaret, reaching over for a cheek kiss and pushing a pair of skis out of the way so I can climb in. "One of the great advantages of having been married to the prime minister was that I got a lifetime pass to Whistler," she explains. Soon she is belting out Adele's "Hello" as we sit in traffic, her voice swooping from octave to octave while bemused college students observe from the sidewalk.

At 67, Margaret, dark-haired and trim in a sleek black dress, still emanates the beauty and ebullience that captured international attention when, in 1971, at the age of 22, she married Pierre Trudeau, Canada's then prime minister. Charismatic and commanding, Trudeau, 29 years her senior, was a popular—and progressive—leader. He was also known as a bit of a playboy (Barbra Streisand, whom he once dated, described him as a mix of "Marlon Brando and Napoléon"). Margaret, though, proved every bit his match. She raised eyebrows and won hearts by refusing to conform to the traditional notions of what a political wife should be. She smoked pot in front of her security detail (when she wasn't trying to ditch them), partied at Studio 54, and was unabashed in her tastes for high fashion, revolutionary art, and rock 'n' roll.

From top: Margaret's infamous night with the Stones, 1977; Margaret with 11-month-old Justin, 1972.

Last fall, Margaret was thrust back into the public eye when 43-year-old Justin, leader of the same Canadian Liberal Party that his father once shepherded, swept to victory in the general election in a confirmation of dynastic manifest destiny. Like Pierre, whose surge of support became known as "Trudeaumania" during his 1968 campaign, Justin, a former schoolteacher, has been heralded as an agent of change for the country after nine years under his Conservative predecessor Stephen Harper.

"Justin is our politician," Margaret says proudly over escargot and vitello tonnato at Maison Boulud in the Ritz-Carlton, oblivious to the eyes of virtually every diner in the restaurant fixed upon her. "He has a deep warmth in him. He wants to know about people, he wants to be inside their minds. His path—whether it is luck or coincidence—is that he is just one of those golden people." He also seems to have inherited her easygoing charm—and, as she points out, her hair ("he certainly doesn't have his dad's," she says).

Margaret's relationship with Pierre was passionate but fraught. "We had this hugely intergenerational marriage," she says. "I was hardly a woman, in my early 20s, and he was a very urbane, sophisticated intellectual in his early 50s." They first met in Tahiti when she was 19 and on vacation with her family; Pierre was then Canada's minister of justice. After a two-year courtship, they shocked the country by getting married in secret in her hometown of Vancouver. Only 13 people attended the ceremony, which was so private that even Pierre's aides were told he'd gone skiing for the week. While some bristled at the age difference, Margaret's parents approved of the union. Her father, James Sinclair, had been a member of the Canadian Parliament himself and, like Pierre, was a staunch Liberal; many in Pierre's inner circle were just happy to see him finally settling down.

"I was a darling wife when I was good, and when I was bad, I was the worst on the planet."

Margaret moved into the prime minister's official residence, at 24 Sussex Drive in Ottawa, and gave birth to Justin just 10 months after the wedding, on Christmas Day in 1971. A second son, Alexandre, arrived on Christmas two years later, followed by a third, Michel, in 1975. Things, at least initially, were good. But the strain of raising a family and balancing the demands of political life began to creep in. Pierre, Margaret says, had expectations of her that she felt she could never live up to. "I was fresh out of university. I was a flower child. I was very free-thinking for my time. I had been raised to be very liberated. My mum only had daughters, and she wanted each of us to be independent," she says. "I loved Pierre deeply. We had a wonderful time when the time was ours and ours alone. But once he married me and got me home and I was having his children, I realized that I had been put in a birdcage." Margaret turned to alcohol and pot and stewed in her resentment. "My husband had all the virtues that a good husband was supposed to have, but he was also dictatorial and old-fashioned. I was always saying, 'What about me? We're in a partnership, aren't we?' I devoted a lot of energy to blaming Pierre." She lowers her voice conspiratorially. "I called 24 Sussex the crown jewel of the federal penitentiary system."

Meanwhile, she stumbled on seemingly simple formalities like what to wear to a White House dinner with Jimmy and Rosalynn Carter—rather than a gown, she wore a knee-length dress, which made headlines in the U.S. and Canada. (The dress was recently in a museum exhibit about fashion and politics in Toronto.) At a state dinner in Venezuela, she decided to honor the country's first lady with an impromptu song instead of a planned toast. (She later admitted that she had taken peyote beforehand.) The press had a field day. "Suddenly I'm in People, and it's all gossip and all about our interior lives," she recalls. "I was aghast."

From left: With Andy Warhol, 1978; Margaret and Pierre as newlyweds, 1972.

In 1977, Pierre and Margaret agreed to separate just as they had married—in secret. "When I finally left Pierre, it had been a long time coming; we tried marriage counselors and everything," she says. Eager to spread her wings, she arranged to leave her three young boys at 24 Sussex in order to take up an apprenticeship with Richard Avedon at his studio in New York, a move widely criticized at the time. "I wanted to become a photographer," she explains. "Pierre had plucked me before I could learn a profession, and I thought this could get me started."

However, any hopes for a quiet split were dashed a few days before her departure when Margaret went to see the Rolling Stones play a private concert in Toronto and wound up hanging out with the band until dawn. "We played dice until about five in the morning, in my hotel suite," she says. "Smoked some dope, talked. It was a good night, and it was my new world. But no one knew I was separated from my husband yet, and it brought a huge scandal." By the time she got to New York, the press had caught wind that something was amiss; upon her arrival at Avedon's studio, a group of reporters had already assembled outside his door. Rumors circulated that she'd had a fling with a member of the Stones, long thought to be Mick Jagger or Ron Wood. "I spent the night with the Rolling Stones, no question, but it was certainly not Mick Jagger. And that's all we'll say about that," she says. (In Wood's 2007 memoir, Ronnie, he wrote of Margaret, "We had a wonderful time and her husband's name never came up.")

Margaret barreled through the late 1970s on a path intended for self-discovery; to an outsider, it looked more like self-destruction. She dated Ryan O'Neal and Jack Nicholson, spent days at Warhol's Factory, and frequently chaperoned Truman Capote home to bed. On the same night in 1979 that her husband's party was crushed in the Canadian election, Margaret—by then publicly separated from Pierre—was photographed dancing ecstatically at Studio 54. The unflattering images appeared in newspapers across Canada. Once an endearing free spirit, she'd become a political punch line; she decided it was time to return home. She moved into a small Victorian house near her husband's residence in Ottawa so she could share custody of their boys. "I needed to protect my life," she says. "I needed to protect my children." She and Pierre officially divorced in 1984.

From left: Margaret with a busboy at Studio 54, 1979; At a dinner in Ottawa with Prince Charles, 1975; With Rolling Stones guitarist Ron Wood, 1977

Margaret now lives in a comfortable two-bedroom flat in the Ville-Marie section of Montreal. The front door opens onto a hallway lined with photographs arranged in mismatched frames, and her kitchen looks well used. A framed to-do list written by John Lennon ("Put back Sean's large mattress" and "Get Margaret Trudeau's book") stands near a photograph of Margaret and Justin, then a toddler, clambering up airplane steps to greet his father. A bank of windows overlooks nearby Mount Royal; Margaret says she walks in the park there every day, rain or shine, to clear her head. She offers me a cookie. "This is me," she says, waving her arm at the living room cluttered with needlepoint pillows and piles of books, though she tells me she did clear away a toy kitchen set and a few errant Legos left by her grandchildren. "You get what you see."

Margaret was diagnosed with bipolar disorder in 2000, and for much of the past decade and a half has devoted herself to raising awareness of mental health issues. She frequently lectures and has written two books addressing the subject. Though her own diagnosis came later in life, Margaret says that she'd long struggled with depression. "I had my first serious bout with mental illness after the birth of my second child," she says. "It was textbook postpartum depression. I was told I had 'the baby blues.' I was 3,000 miles away from my support system, which had always been my family, and my husband criticized me daily. I was alone. I just thought I was going to live my life as this very desperately sad person, who wept uncontrollably." She and Pierre agreed that she should seek medical help, but at the time, in 1974, "no one was yet talking about manic depression, as bipolar illness was then called," she says. A girlfriend quietly took Margaret to the Royal Victoria Hospital in Montreal, where she was medicated, but she left without a clear understanding of what was wrong.

Margaret's waves of sadness were often followed by soaring highs—a condition only exaggerated by her wealth and social position. "When I was manic, it was grand mania," she says. "Where someone else might have run off with the guy from the 7-Eleven, I ran off with the Rolling Stones. I would spend all my money buying Birkin bags; somebody else would have spent all the grocery money. It's paralyzing either way. You don't have the ability to have a second, sober thought." She took lithium for a time, but stopped because the medication made her gain weight.

"Where someone else might have run off with the guy from the 7-Eleven, I ran off with the Rolling Stones."

Her son Michel's death in an avalanche during a ski trip in 1998 proved a tragic tipping point. Margaret was devastated; the pain of losing a child proved too much to bear. She can remember imploring her doctor to put her into a medically induced coma, "just to make it stop," she says. "I couldn't deal with it." Her marriage to her second husband, real estate mogul Fried Kemper, with whom she'd had two more children—a son, Kyle, and a daughter, Alicia—had already begun to unravel, and ended the following year. Pierre, who was suffering from Parkinson's disease, succumbed to cancer not long after—by then he and Margaret had repaired their relationship, and she was at his bedside during his final days. In the months that followed, she dropped 30 pounds and refused to leave the house; her family staged an intervention, which led to her hospitalization and diagnosis.

Margaret now monitors herself closely for signs of imbalance. "A big part of being healthy is making the choice not to be addicted to the mania," she says. If she feels an episode coming on, she declares a "lock-down" day, ups her medication, and stays home. "I don't make decisions. I don't get in my car. This one day for me is how a lot of mentally ill people live every day of their lives."

She is close to all of her kids, including Justin, who checks in regularly. Her daughter, Alicia, lives nearby, and they see each other almost daily. Margaret maintains that she won't marry again. "I was a darling wife when I was good, and when I was bad, I was the worst on the planet," she says. She also refuses to allow age to panic her: "I am almost shocked to see that I have grown old. But I am also amused." (Her anti-aging advice: "Get a 25-watt pink bulb and install it in your bathroom.")

From left: Escorted by Richard Nixon to a concert in Ottawa, 1972; At Justin's swearing-in ceremony with his wife, Sophie Gregoire Trudeau, and their children, 2015; With Pierre, Elizabeth Taylor, and John Warner in Washington, D.C., 1977

Last Christmas, her life came to a full—almost spiritual—circle. She and her children were at Harrington Lake, the prime minister's country retreat in Quebec. There was a rare full moon on Christmas Day; she had been in the same place for the last one, in 1977. "The grandchildren and I were jumping on each other's shadows in the snow," she remembers. "The men—my boys—were playing with swords in the moonlight. It was dream stuff, at this magical place where I had spent so much of my early life with them, and I thought, 'We're back.' I feel I have been given a second chance—who gets a second chance?"

This article originally appeared in the April 2016 issue of Harper's BAZAAR.