The following season, 1990, Armstrong notched ten sacks, the first of five seasons he reached double-digits. He was NFC Defensive Player of the Month in September 1990, in which he totaled twenty-five tackles, five sacks, two forced fumbles, one fumble recovery, and one pass defended.
In 1991, he recorded only 1.5 sacks, in part because although he was the starting left defensive end he moved to defensive tackle in the Bears' "nickel defense," perhaps cutting down on his pass-rush opportunities. He was slated to play that spot again in 1992, however, the development of Alonzo Spellman allowed Armstrong to play end in all situations. As a result, his sack total was 6.5.
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. In 1993, Armstrong notched 11.5 sacks and forced three fumbles. He was named NFC Defensive Player of the Week for three tackles, one forced fumble, two fumble recoveries, and two sacks in a Thanksgiving Day win over Detroit. He also had six tackles and two sacks at Philadelphia on October 10, 1993, and then a career-best 2.5 sacks at Kansas City on November 21, 1993.
In 1994, his last in Chicago, he had 7.5 sacks. In the playoffs that season, against the Minnesota Vikings, on January 1, 1995, Armstrong recorded both of the Bears' sacks in a 35�18 win over the Vikings for which he was awarded the NFC Defensive Player of the Week award.
He then went on to play for the Raiders and Dolphins, where he continued to gain accolades and praise for his performance until he retired after 2003. He was president of the NFLPA for eight years while he was playing, He also worked as an agent for Penn State and currently lives a retired life with his wife and kids in Florida.