Alexander Semin notched his fourth career hat trick, Mathieu Perreault added two assists and Michal Neuvirth stopped 29 shots for the Caps, who remained unbeaten in their last eight games at the Verizon Center.
The second of Evander Kane's two goals for the Thrashers tied the game 3-3 with only 32.9 seconds left in regulation.
Mason ended up making 40 saves in Atlanta's loss.
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