After graduating from Peekskill High School, Armstrong started off slowly as a college athlete, averaging under 4 points in each of first 3 seasons at UConn with the Huskies. However, he greatly improved in his senior year, averaging 9.7 points, 6.6 rebounds, and 3.1 blocks, and shooting 61% from the field. He followed after his teammate Josh Boone, and won the Big East Defensive Player of the Year in the 2005–06 season.
He was declared eligible for the 2006 NBA draft, and was selected by the New Orleans Hornets with the 12th overall pick. Known for his shot-blocking and athleticism, he was anticipated by analysts to be an instant contributor to the Hornets front line, but the team then traded for the Chicago Bulls center Tyson Chandler. Chandler was named the starting center, and Armstrong did not do much to improve in the following off-seasons.
On January 11, 2010, Armstrong was traded to the Sacramento Kings for a conditional 2016 second-round draft pick.
On February 18, 2010, Armstrong was traded to the Houston Rockets, along with Kevin Martin, for Carl Landry and Joey Dorsey. He was waived by the Rockets on April 10, 2010.
Armstrong signed with the Washington Wizards on July 13, 2010.
On February 23, 2011, Armstrong was traded to the Atlanta Hawks, along with Kirk Hinrich, in exchange for Mike Bibby, Jordan Crawford, Maurice Evans and a first-round pick in the 2011 NBA draft.
In July 2011, Armstrong signed with ASVEL Basket of the French Pro A League. In August 2012, he signed with Panathinaikos of the Greek Basketball League. In December 2012, he left Panathinaikos.
On January 2013, Armstrong was acquired by the Santa Cruz Warriors of the NBA D-League. He played very well for the Warriors, earning 2nd Team All D-League Defensive Team honors. In May 2013, he joined Changan Group Guangdong of China for the 2013 NBL season.
On September 27, 2013, he signed with the Indiana Pacers. However, he was waived on October 26.
In November 2013, he was re-acquired by the Santa Cruz Warriors.
On December 11, 2013, after playing in six games with Santa Cruz, Armstrong signed with the Golden State Warriors. On December 29, 2013, he was waived by Golden State. On January 3, 2014, he was re-acquired by Santa Cruz.
On February 3, 2014, Armstrong was named to the Futures All-Star roster for the 2014 NBA D-League All-Star Game.
On February 22, 2014, he signed a 10-day contract with the Golden State Warriors. On March 4, 2014, he returned to Santa Cruz after his 10-day contract expired. On March 30, 2014, he signed another 10-day contract with the Golden State Warriors. On April 9, 2014, he signed with Golden State for the rest of the 2013–14 season. He started the final game of the regular season and recorded a double-double. On July 30, 2014, he was waived by Golden State. While playing for Golden State, Armstrong became the first player in NBA history to wear jersey number 57.
On August 6, 2014, he signed a one-year deal with Beşiktaş of the Turkish Basketball League.
On July 30, 2015, he signed with Tüyap Büyükçekmece also of Turkey.
Armstrong appeared as himself in the 2015 independent film Four Square Miles to Glory.