He signed his first NBA contract in 1995 with the Orlando Magic. In nine years in Orlando, the team with Armstrong never recorded a negative record this season. 2003 Armstrong wrote in the New Orleans Hornets. In December 2004 he was traded from there to the Dallas Mavericks for Dan Dickau. After the role as a finalist in the 2006 NBA Finals with the Mavericks, he was sent in exchange for Anthony Johnson to the Indiana Pacers.
After the end of his playing career Armstrong was assistant coach of the Dallas Mavericks.