Long's acting coach was Betty Bridges, better known as the mother of "Diff'rent Strokes" star Todd Bridges. Her earliest role was in the Disney television movie, "The B.R.A.T. Patrol" alongside Sean Astin, Tim Thomerson and Brian Keith. Her first notable role on television was a three-year contract role as Kathryn "Kat" Speakes on the soap opera "Guiding Light". Long portrayed Kat from 1991 to 1994. Long played Brandi in "Boyz n the Hood". The film excited Long, who was doing her "first real movie role". It helped Long build her confidence as an actress. "It introduced me to the world in a way that it was okay for me to be who I am and still find success," she added. "I didn’t have to conform to anything." From 1994 to 1995, she played Will Smith's girlfriend and fiancée Beulah "Lisa" Wilkes on "The Fresh Prince of Bel-Air". Jada Pinkett Smith was originally supposed to play Lisa, but was too short for the role (opposite a 6'2" Will Smith), thus leaving Nia to take the part. However, in the second season of the show (episode 8 - “she ain’t heavy” 1992), Nia appeared as a love interest of Will’s, ‘Claudia’ along side Queen Latifah. 2000, Long was offered the role of Alex Munday in "Charlie's Angels", but she turned it down, and the role was eventually given to Lucy Liu. In 2003, she joined the cast of the drama "Third Watch", where she played NYPD Officer Sasha Monroe, continuing until the series finale in 2005. In 2005 and 2006, Long appeared on "Everwood", and appeared on "Boston Legal" during its 2006–2007 season. Long also starred in "Big Shots" from 2007–2008 alongside Michael Vartan and Dylan McDermott. In 2016, Long was cast in a recurring role for the TV series Empire. She acquired the role of Giuliana, a Las Vegas club owner.