Armstrong scored on a wrist shot in front of the net on a pass from rookie Evgeni Malkin, who had a goal and two assists.
Sidney Crosby had a goal and an assist as the Penguins avoided matching their longest losing streak of the season.
Marc-Andre Fleury stopped 20 shots as the unusually listless Thrashers were outshot 43-23 and never led.
Kari Lehtonen made 39 saves for the Thrashers.
The Southeast Division-leading Thrashers have lost two in a row after winning a franchise record-tying five straight games and seven of eight.