Union moved at the age of eight to Pleasanton, California. She proved to be a talented athlete in high school, excelling at basketball, soccer, and track & field at Foothill High School, where she was known as Nikki. During high school, she dated current NBA star Jason Kidd.

In 1992, Union was assaulted and raped at gunpoint while working at a shoe store. Her attacker fled, but later turned himself into the police after another rape. He took a plea bargain and received a 33-year jail sentence; Union has said she will testify at his parole hearings to ensure he's not released early. In 2006, Union visited Congress for the National Domestic Violence and Sexual Assault Legislative Action Days.

Union originally intended to become a lawyer. 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.

With the 2000 release of Bring it On, Union gained wide exposure. Her new fame landed her on magazine covers and talk shows, and she gained popularity among movie fans.

She also made a guest appearance in Star Trek: Deep Space Nine as a Klingon named N'Garen (in the episode "Sons and Daughters"). Her most recent role was in the short-lived 2005 ABC series Night Stalker. She has also starred in the independent drama films Neo Ned and Constellation.