A first-round (12th overall) pick in the 1989 NFL draft, he played for the Chicago Bears, Miami Dolphins and Oakland Raiders.
Armstrong was an all-state selection and was named one of The Birmingham News' top twelve players in Alabama as a high school senior.
As a sophomore in 1986, Armstrong was a back-up with two starts on the 10–1–1 Sun Devils that finished fourth in the AP poll and defeated the Michigan Wolverines in the Rose Bowl.
As a junior starter in 1987, Armstrong had a key role in the defeat of the Oregon State Beavers, when he sacked Erik Wilhelm in the end zone for a safety that began an 11–0 scoring run by the Sun Devils in sealing a 30–21 win.
Armstrong was denied a final year of eligibility by the NCAA due to an "academic mix-up," which he could only recoup if he transferred to another school.
[3] He finished his college career on the sidelines in the Gators' 14–10 win over the Illinois Fighting Illini in the All American Bowl on December 29, 1988.
After reporting late to training camp because of a contract dispute, and then struggling through the first two games of the season, Armstrong finally found his niche at left defensive end against the Lions, making five solo tackles, defending a pass and getting his first pro sack by dumping Lions quarterback Bob Gagliano.
His teammates voted him the winner of the Brian Piccolo Award for "courage, loyalty, teamwork, dedication, and sense of humor."
On March 16, 1993, Armstrong re-signed a three-year $3 million deal with the Bears which was reported to make him one of the five highest-paid players on the team.
Armstrong sustained an Achilles tendon injury on September 30, 2001, causing him to miss the final thirteen games of the 2001 season.
In 2002 and 2003, Armstrong was pressed into a starting role due to injuries of the so-called "run down defense" of the Raiders.
In 2002 it was a groin injury that put him in the injured-reserve list and in 2003, he injured a shoulder in November which ended his 2003 season after ten games.
According to NFLPA records, in his fifteen NFL seasons Armstrong earned approximately $23 million in salary and bonuses.
In 2009, Armstrong stood for election to become the full-time executive director of the NFLPA, a position left vacant by the death of Gene Upshaw.
He is also the agent for several broadcasters such as Peter Gammons, Chris Mortensen, Dan Le Batard, and Stugotz (John Weiner).