Florida jumped a point in front of the Capitals for first place in the Southeast Division.
The Panthers have lost three more games than they have won this season, but they lead the league with 11 overtime losses.
It was a 1-1 contest heading into the third, with Samuelsson having scored 33 seconds deep into the middle period and Brooks Laich tying it midway through the stanza. Samuelsson then put Florida in front for good on the power play 4:57 into the third and Stephen Weiss added to the lead by tipping in a shot.
John Carlson brought the Capitals within one with 2:21 remaining, but Shawn Matthias scored into an empty net in the final minute.
The win went to Scott Clemmensen, who made 26 saves. Michael Neuvirth stopped 34 shots for Washington, which has lost four out of five.