Jagr scored off a three-on-one break just under two minutes in the game after trading passes with Kip Miller, sparking a five-goal outburst. Just over two minutes later, Jagr set up Robert Lang for a one-timer that made it 2-0.
Jagr and Miller have combined for 18 goals and 32 assists in 16 games since they were placed on a line with Michael Nylander.
Former Capitals farmhand Byron Dafoe faced just 13 shots and was replaced by Pasi Nurminen to start the second period.
Vyacheslav Kozlov spoiled Olaf Kolzig's shutout nine minutes into the second with his 14th goal. But Kolzig made 26 saves as Washington stretched its road unbeaten streak to four games (2
The Thrashers fell to 7-4-1 under new coach Bob Hartley and are 13 points out of the final Eastern Conference playoff berth.