Union originally intended to become a doctor in college. She attended the University of Nebraska before moving on to Cuesta College. She eventually transferred to UCLA and earned a degree in Sociology; while studying there, she interned at a modeling agency to earn extra academic credits. She was asked by the agency to sign on with them as a new model. She needed money to pay off some college loans and debts, so she eventually agreed to become a fashion model. Union started her acting career in minor roles. Most were in teen movies like 10 Things I Hate About You and Love and Basketball. She also appeared in the television show Sister, Sister as Vanessa. In 2000, Union landed the role of Isis in the cheerleading movie Bring it On opposite Kirsten Dunst. Bring It On helped push Union out into the mainstream and she began gaining more exposure. This led to Union being cast in the CBS television drama City of Angels as Dr. Courtney Ellis.

Union was cast in her first leading role in the 2003 film Deliver Us From Eva with rapper L.L. Cool J. This was her second time working with the rapper since making a cameo in his video "I Need love" in 1995. The film received fair reviews from critics and it showed that Union was a leading lady. But first, Union landed the role of Oscar nominee Will Smith's girlfriend Syd in the film Bad Boys II, a box office success grossing over $273 million worldwide. Union then starred with Academy Award winner Jamie Foxx in the film Breakin' All the Rules in 2004. Union starred in the short-lived 2005 ABC series Night Stalker. She has also starred in the independent drama films Neo Ned and Constellation, the latter of which was released to theaters. She also starred in the 2005 remake of The Honeymooners with comedian Cedric The Entertainer. In 2006, she starred as Busta Rhymes' love interest in the music video for Rhymes' "I Love My Chick". Union starred in the 2007 films Daddy's Little Girls by Tyler Perry (released on Valentine's Day) and the Christmas film The Perfect Holiday which opened on December 12.

In an interview with Art Nouveau Magazine, Union complained about the lack of roles for black actresses and actors in Hollywood: "There used to be [roles] specifically written black, if you knew Denzel was doing a movie you knew his wife, girl or love interest was going to be black [but] that’s not necessarily the case anymore. You’re in that room with every amazingly talented actress of every hue, and it’s a dogfight, it’s hard". In 1996-2007, she appeared as Keesha Hamilton on 7th Heaven. In 2008, Union appeared on Ugly Betty for three episodes (36-38) as Renee, Wilhelmina Slater's (Vanessa L. Williams) sister and Daniel Meade's (Eric Mabius) love interest. She also made a cameo appearance in the music video for Ne-Yo's "Miss Independent". In 2009, she joined the cast of the U.S. television series Life on NBC.

• The Body Politic (2009) (TV) (pre-production) (rumored)
• The Van Zandt Shakedown (2009) (pre-production) (rumored)
• Body Politic (2010) (TV) (filming) (rumored)
  
• "Life" .... Jane Seever (3 episodes, 2009)
       - Initiative 38 (2009) TV episode .... Jane Seever
       - 5 Quarts (2009) TV episode .... Jane Seever
       - 3 Women (2009) TV episode .... Jane Seever
• Cadillac Records (2008) .... Geneva Wade
• "Ugly Betty" .... Renee Slater (3 episodes, 2008)
       - Burning Questions (2008) TV episode .... Renee Slater
       - Twenty-Four Candles (2008) TV episode .... Renee Slater
       - A Thousand Words by Friday (2008) TV episode .... Renee Slater
• Meet Dave (2008) .... No. 3 – Cultural Officer
• The Perfect Holiday (2007) .... Nancy
• The Box (2007/II) .... Det. Cris Romano
• Daddy's Little Girls (2007) .... Julia
  ... aka Tyler Perry's Daddy's Little Girls (USA: complete title)
• Football Wives (2007) (TV) .... Chardonnay Lane
• Running with Scissors (2006) .... Dorothy
• "Night Stalker" .... Perri Reed (10 episodes, 2005-2006)
       - What's the Frequency, Kolchak? (2006) TV episode .... Perri Reed
       - Timeless (2006) TV episode .... Perri Reed
       - Into Night (2006) TV episode .... Perri Reed
       - The Sea (2006) TV episode .... Perri Reed
       - The Source (2005) TV episode .... Perri Reed
        (5 more)
• "Family Guy" .... Shauna Parks (1 episode, 2005)
  ... aka "Padre de familia" (USA: Spanish title)
       - Peter's Got Woods (2005) TV episode (voice) .... Shauna Parks