Armstead was an All-Rookie player, but he was not a starter in his first three seasons, and the Linebacker did not see as much action as he would have liked. In 1996, Armstead started at Left Linebacker, and the former Hurricane would see him pile up the tackling stats. Armstead had five straight 100-Tackle years (1996-00) and was a Pro Bowler from '97 to '01.
In 1997, he was a First Team All-Pro, but he arguably had his best season in 1999, where he was a Second Team All-Pro. That season, Armstead led the NFL in the new category of Tackles for Loss (21), and he had career-highs in Sacks (9.0) and Approximate Value (21).
Armstead's last Pro Bowl year (2001) was his last in New York, as he signed with the Washington Redskins, where he played two more years. With the Giants, Armstead had 30.5 Sacks and 778 Tackles.