Etienne Capoue capitalised on Mawson's sloppiness to score the only goal of a dull encounter, the 1-0 win taking Watford onto 40 points and all-but securing their top-flight status.
But it was a fifth loss in six games for the Swans, who are swimming against the tide in their bid to avoid the drop.
Paul Clement's side remained 18th in the table and two points behind Hull, who lost at Stoke, with just five games left to turn their season around.
A goalless draw at home to fellow bottom three side Middlesbrough represents the only point for the south Wales outfit since a win over Burnley at the start of March.
Things started brightly for Clement's men in Hertfordshire with Heurelho Gomes making a smart save from Gylfi Sigurdsson as the Swansea skipper had the game's first decent chance.
But Gomes was at fault as the visitors passed up a glorious opportunity to take the lead with a little over 10 minutes on the clock.
The Watford goalkeeper flapped at a Sigurdsson free-kick with the ball looping towards Federico Fernandez, but the defender could only head over from six yards with the goal at his mercy.
Fernandez fouled M'Baye Niang on the edge of the Swansea box but the AC Milan loanee could only drill the resulting free-kick straight at Lukasz Fabianski.
The Swansea stopper had to be at this best moments later as Troy Deeney looked to sweep home Daryl Janmaat's cross only to see his effort tipped over by the Poland international.
Sebastian Prodl's goalbound header from the corner that followed was deflected behind as the hosts finally started to come alive in the contest.
As the Hornets flew forward, they were almost caught out from a pacey Swansea counter-attack, although Ki Sung-yueng could only shoot straight at Gomes at the end of the move.
Watford were ahead four minutes before the interval as Capoue caught Mawson napping in possession, robbing the Swansea defender before turning home at the second attempt.
The second half offered little quality and even less in the way of clear-cut chances with Watford not much better than their relegation-threatened visitors.
Abdoulaye Doucoure saw a long-range strike fly wide of the angle of Fabianski's goal, with Fernando Llorente heading wide when well-placed as Swansea chased an equaliser.
Walter Mazzarri introduced Stefano Okaka from the bench and the Italy international thought he had wrapped up the points moments later with an acrobatic finish, only to be flagged offside.
Gomes did well to clear a Sigurdsson free-kick from under his crossbar but in truth Swansea never threatened to grab a goal to move them closer to Hull after the Tigers lost at Stoke, with Mark Hughes' men the visitors to the Liberty Stadium next weekend.