Mirojnick was born in New York City, to Barry Mirojnick and Ellen Mirojnick, an Actress and costume designer (nominated for a BAFTA for Best Costume Design for the film 1992 Chaplin). Her maternal grandparents were Abraham "Abe" Schneid and Sunny (née Schneider). Her parents divorced in 1987. She is of Eastern European and Mediterranean descent. Lili attended The Buckley School in Sherman Oaks, California, after attending the American Academy of Dramatic Arts (AADA) in New York, New York.
Starting in 2005, Mirojnick started appearing in uncredited or minor roles in films, before landing a lead role in the independent film Kill the Habit (2010). From 2011 through 2014, she had a number of guest appearances in episodes of network television series, including Person of Interest, Blue Bloods and Grey's Anatomy.
Mirojnick is starring in a lead role, as Det. Meredith McCarthy, in the Syfy series Happy! (2017).
