In-N-Out heiress Lynsi Snyder's $3.6 billion fortune hasn't stopped her taking up a side hustle. Photo / Getty Images

Notoriously private billionaire CEO Lynsi Snyder has spoken about her troubled life and her unexpected side job during a very rare media interview.

The 37-year-old is the heiress to the In-N-Out burger chain fortune, with a net worth of around $3.6 billion, according to Forbes.

The beloved US fast food chain was founded by her grandparents Harry and Esther Snyder in California in 1948, reports News.com.au.

Due to a series of family tragedies, Ms Snyder became the family's last remaining heir in 2000 when she was just 18, after the deaths of her father and uncle.

She began gradually receiving stakes in the business as part of a complicated trust plan made by her grandparents, and in 2017 she received the last slice of her fortune, on her 35th birthday.

She took over as president of the company in 2010, but despite being one of America's richest — and youngest — people, Ms Snyder has long shunned the spotlight in favour of a more reclusive life.

However, she's just given an extremely rare interview with an obscure publication, revealing fresh details of her tragic life — and her side hustle.

The mother of four — who has been married four times — told The Christian Post she previously battled drug and alcohol abuse before turning her life around due to her faith.

"I have learned so much through my broken relationships along with the ups and downs that come through marriage," she told the publication.

"I've been the one to hurt, and I've been hurt. I have gained insight and growth through both sides of the coin."

She also spoke about her side job as a Christian minister, after founding the Army of Love ministry during a particularly "painful" period of her life.

"I was calling out to God as my third marriage was failing. I was in a place where I felt I couldn't do ministry because my own heart and home were a mess," she told the Christian Post.

"My (ex) husband and I were in constant turmoil. I became desperate for the hope that I could be used by God despite my circumstances."

She ended up studying discipleship online and filed for a non-profit tax-exemption.

As she waited for it to be approved, she ended up meeting her current husband Sean Ellingson, and the filing was approved shortly after the pair tied the knot.

Ms Snyder said she was also a believer in "servant leadership" — a leadership philosophy where the leader's main goal is to serve others rather than grow the business.

She said she worked hard as In-N-Out CEO to "maintain" the business without "compromising the quality of product, service, or standards."

"My grandparents set the bar high and I only try to raise it," she said.

"All families have their issues and we aren't perfect, but we try really hard and there are so many good-hearted people here. I love my job."

One of the last times Ms Snyder spoke so candidly was back in 2017, when she gave a tell-all interview to I am Second, a not-for-profit organisation promoting Christianity.

"After my dad died, my world shattered. I longed for attention, something to fill the huge void," she said at the time.

Her father Guy Snyder died of an overdose in 1999, when his daughter was just 17.

Her uncle Richard Snyder had died in 1993 in a plane crash along with In-N-Out executive vice president Philip R West.

She also spoke about the impact of her father's drug addiction during her childhood.

"It was really hard for me to see him fail and be weak, because I knew how bad he wanted to be a good husband and good father," she told the organisation.

She said her parents had divorced when she was 12, after the drugs and "another woman" impacted on the marriage.

"That's when I really started longing for that attention and that love, because my dad was the greatest source of that," she said.