Union had a prominent role as Rachel Meyers in Almost Christmas, released in November 2016.
Ariel Scotti of The New York Times panned Union's performance: "Each overused phrase that falls out of her character, Rachel's, vindictive, childish mouth takes viewers further out of the movie experience." In April 2017, Union announced her first book, a memoir, entitled We're Going to Need More Wine.
She also appeared in Sister, Sister as Vanessa, in Smart Guy as Denise, and in five episodes of 7th Heaven as Keesha Hamilton. In the same year, she performed the role of Isis in the cheerleading movie Bring It On, opposite Kirsten Dunst. It led to Union being cast in the CBS television drama City of Angels as Dr. Union was featured in The Brothers and was seen as having a "beguiling sincerity, even when she's fudging the truth." Union was cast in her first leading role in the 2003 film Deliver Us from Eva with LL Cool J.
She then met with series creator Frank Spotnitz and executive producer Daniel Sackheim, who told her they thought of her anytime they thought of the character.In June 2011, Union defended the music video for the Rihanna single "Man Down" after it sparked controversy for Rihanna depicting a rape victim shooting and killing her attacker.Union related that every rape victim was "unique", which extended to how they believed justice should be carried out.In 2013, Union began starring as lead character in the BET drama series Being Mary Jane, for which she has received critical acclaim and an NAACP Image Award.She co-starred in the 2016 film The Birth of a Nation, and next appeared in Almost Christmas (2016) and Sleepless (2017).