Rooney Mara first worked with Joaquin Phoenix on the set of 2013's Her. The movie follows a man who falls in love with a computer operating system, and Mara portrayed Phoenix's ex-wife in the movie.

Phoenix opened up about meeting Mara to Vanity Fair in a 2019 interview, telling the magazine, "She's the only girl I ever looked up on the internet." He continued, "We were just friends, email friends. I'd never done that. Never looked up a girl online." During filming, however, Phoenix also said he thought Mara despised him, but later learned she was just shy. And she actually liked him, too. 

The actors reunited in a professional setting to shoot Mary Magdalene, in which Mara played the titular role, while Phoenix portrayed Jesus. According to W magazine, Mara and Phoenix were just friends during production, but they started dating after the film wrapped. While dating rumors had begun to swirl on the set of Mary Magdalene, it wasn't until the Cannes Film Festival in 2017, when they revealed their romance to the world.

After three years of dating, the couple became engaged in 2019, according to Us Weekly.