Armstrong first signed with the NBA as a free agent for the Orlando Magic in late 1994-95, playing just 3 games with 10 points and 8 minutes of action. In 95-96 he played just 13 games in 41 minutes, scoring 42 points total.

He saw 67 games in his first full season on the roster in 1997-98, averaging 6 points per game in 15 minutes per game off the bench. Armstrong won the NBA Sixth Man of the Year Award and the NBA Most Improved Player Award in 1999, thus becoming the first player in NBA history to win both awards simultaneously. In a 1999 game against the Philadelphia 76ers, Armstrong stole an inbounds pass and streaked to the other end of the court for a game winning layup as time expired. He subsequently became the starting point guard for the Magic. His career year was in 1999-00, averaging 16.2 ppg in 31 mpg. During his nine years in Orlando, the team never posted a losing record, making the post-season seven times.

On July 7, 2003, Armstrong was arrested after an incident outside an Orlando night club. He was subsequently charged with resisting arrest and assaulting a police officer, but the case was eventually dismissed.[3]During that off-season, Armstrong signed with the New Orleans Hornets as a free agent. He was traded by the Hornets to the Dallas Mavericks in exchange for Dan Dickau and a second round draft pick on December 3, 2004.

On December 19th, 2005, while he was still with the Dallas Mavericks, Armstrong was fined $1,000 for grabbing a microphone before a Mavs game against the Minnesota Timberwolves at the American Airlines Center and yelling "How 'bout those Redskins!" Only a few hours prior, the Cowboys had been routed by the Redskins 35-7. Armstrong was raised in North Carolina as a Redskins fan.[4]After appearing in the 2006 NBA Finals with the Mavericks, he was traded to the Indiana Pacers in exchange for guard Anthony Johnson in July 2006.

Armstrong was released by the Pacers on October 1, 2007, and signed with the New Jersey Nets after clearing waivers.[5][6]He appeared in 50 games in 2007-08, averaging 2.5ppg in 11.0 minutes, and buried three 3-pointers in his final appearance of the season. After retiring, Armstrong re-joined the Mavs, as an assistant coach.