The centre-half's sloppy control on the edge of his own box allowed Etienne Capoue to rob him and fire past keeper Lukasz Fabianski.
The Welsh side have now taken just one point from their last six outings and remain two points adrift of safety with only five games left to play.
Etienne Capoue's goal on his return from injury proved the difference as Watford beat ailing Swansea 1-0 on Saturday (April 15) in their Premier League clash leaving the losers without a win in six games.
Etienne Capoue capitalised on Mawson's sloppiness to score the only goal of a boring encounter, the 1-0 win taking Watford onto 40 points and all-but securing their top-flight status.
"It was a better performance than last week at West Ham, more spirit and confidence in the players but not the quality where it was needed".
Their sole consolation from another tough away afternoon came in the form of 17th-placed Hull City also losing, beaten 3-1 at Stoke City.
Moments later the vistors went close again, when Federico Fernandez's header landed on the roof of the net after Gomes had miscued his attempt to punch away a Sigurdsson free-kick.
Soon after the restart, Sigurdsson got a strike away to signal Swansea's intent for the second half, but Watford were delivering another solid performance under Mazzarri and Janmaat nearly made it two with a powerful effort from distance.
The goalmouth action dried up in the final quarter, however, with the hosts content to drop deep, sit on their lead and keep Swansea at arm's length.
- Watford have kept three consecutive clean sheets on home soil in the Premier League for the first time since September 2015. Only two goals have been scored in the three Premier League meetings between the two clubs, with the match at the Liberty Stadium earlier in the season ending in a stale goalless draw.
Clement has revealed that midfielder Jack Cork, who has played in every game since he was appointed, will miss the Watford trip as a result of the sprained ankle he suffered at West Ham last weekend. And while there is still very much a chance of survival, the Swans' manager believes it will come down to a head-to-head scrap with Hull City, who go into the latest round of matches two points ahead in the league standings.