Her career began in the 1990s, when she made dozens of appearances on television sitcoms, prior to landing supporting roles in 1999 teen films She's All That and 10 Things I Hate About You.

She also had starring roles in the CBS medical drama series City of Angels (2000) and in the films Bad Boys II (2003), Cradle 2 the Grave (2003), Neo Ned (2005), Cadillac Records (2008), Top Five (2014), Breaking In (2018), and The Perfect Find (2023).

"[8] According to Union, her mother taught her to have a "world perspective" and took her to a gay pride parade at the age of eight after the family moved to Pleasanton, California.

[9] She attended Foothill High School[10] where she was a year-round student athlete, competing in Varsity soccer, track, and basketball.

"[16] During the summer before starting her sophomore year of college at UCLA, at the age of 19, Union was attacked and raped at gunpoint at her part-time job at a Payless shoe store by a robber.

[17] Union has stated that she would not have survived the attack had it not been for self-defence lessons she had learned by watching Oprah Winfrey's talk show.

When casting Eva, director Gary Hardwick was looking for an actress capable of instantly changing "from funny to caustic and dramatic."

[25] In 2003, Union landed the role of the main character Mike Lowrey's girlfriend, Syd, in the film Bad Boys II, a box-office success grossing more than $273 million worldwide.

She and co-star Jeremy Renner were noted by Mark Olsen of the Los Angeles Times as having "a strange, offbeat chemistry that drives the film".

"[40] In 2008, Union appeared on Ugly Betty for three episodes as Renee, Wilhelmina Slater's sister and Daniel Meade's love interest.

[50] Union joined the cast of the American television series Life on NBC and appeared in four episodes prior to its cancellation in May 2009.

[citation needed] After she was reported to be joining the cast in June 2010, Union appeared in episodes of Army Wives and NTSF:SD:SUV:: in 2010 and 2011.

[53] In June 2011, Union defended the music video for the Rihanna single "Man Down" after it sparked controversy for its depiction of a rape victim shooting and killing her attacker.

[67] She believed the film would do well[68] and faced comparisons to her character, who was a newlywed while Union was engaged at the time, a similarity she dispelled by insisting she kept her relationship "enjoyable, fresh and exciting".

[73] In October 2016, Union was featured in The Birth of a Nation as Esther, a slave in the Antebellum South who is raped by a white man.

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 2019, Union began starring as Syd Burnett in the Bad Boys spin-off L.A.'s Finest, a Charter Spectrum original series that premiered May 13 [80] and was later renewed for a second season.

[83] On Disney Investor Day, December 11, 2020, it was announced that Union would be starring with Zach Braff in a remake of Cheaper by the Dozen,[84] which was released under the same title in March 2022.

[85] In December 2021, she played Tootie Ramsey in The Facts of Life segment of the third edition of Live in Front of a Studio Audience, recreating the episode "Kids Can Be Cruel".

The book "feature[s] personal stories and reflections on a range of topics that continue to define the contemporary landscape: sexuality, womanhood, friendship, race, marriage, and beauty.

[92] In May 2020, Union released her first children's book, Welcome to the Party, an ode to newborns and non-traditional families that was inspired by her daughter.

The collection is affordably priced and “empowers consumers to customize a regimen specific to their texture and style preference.”[99] That same year, she joined Bitsy's as a cofounder with “the goal of making allergen-friendly, school-safe snacks that are accessible and affordable for all families regardless of their socioeconomic or geographic status.”[99] Union and her daughter Kaavia are a part of the ownership group of Angel City FC of the National Women's Soccer League.

"[58] In the months following Martin's death, Union supported a petition that called for Florida District Attorney Norman Wolfinger to bring charges against George Zimmerman.

"[107] Union rebuked Todd Akin's position on abortion, which he defended saying that a woman would not be able to get pregnant in the case of "legitimate rape.

"[112] Union ran in the Global Race for the Cure in Washington, D.C., on Saturday, June 2, 2012, in honor of her friend Kristen Martinez, who died from breast cancer.

[115] Union became a spokesperson for Planned Parenthood's breast health initiative and launched the Women Are Watching campaign with other actresses in 2012.

[116] In 2020, Union collaborated with JusticeLA to create a public service announcement #SuingToSaveLives about the health of people in L.A. County jails amid the COVID-19 pandemic.

[125] In November 2014, the office of Atlanta Mayor Kasim Reed announced a campaign titled "Take a Stand" and its commission of a short film featuring Union and Tika Sumpter.

[130] Union said that when she got divorced she "sort of realized that she hadn't been making sound choices which were the best for her to pursue her hopes and dreams and aspirations and passions.

[136] Union stated in an interview prior to the marriage that she and Wade would be signing a prenuptial agreement to protect their individual assets.