Watford entered the match on the back of a five-game unbeaten run and made a promising start when Don Cowie fed Danny Graham for the striker's sixth goal in six games.
But Australian striker Scott McDonald quickly equalised, firing in Barry Robson's right-wing cross, and Andrew Taylor added a second when goalkeeper Scott Loach parried a Robson shot to him.
The afternoon got even worse for visitors Watford when Mariappa was sent off late on. The central defender received a second yellow card in the 88th minute after hauling down Marvin Emnes.
The defeat, which means Watford drop a place to ninth in the Championship table, arrested the momentum established by the unbeaten run and leaves the Hornets four points off a play-off spot.
Mariappa, 24, came through the youth ranks at Watford and has made over 160 appearances for the club, finding the net three times.
He featured in Watford's most recent season in the Premiership in 2006-07 and was handed the captaincy during parts of 2009-10.
He was included in an extended Fiji national training squad for the Pacific Games but it is unclear whether he will ever line up for the nation.
Fiji Football Association technical director Saiyad Ali says Mariappa has shown an interest in representing Fiji and believes his involvement would be a big boost for the development of football in the country.
"Adrian is wanting to come to Fiji for a visit next year," Ali told The Fiji Times last December. "He wants to see where his grandparents and father are from. What I have suggested is for Adrian to come earlier so we can use him to also promote the sport in Fiji."
Mariappa is not the only overseas-based star the Fiji FA have their eye on.
Canadian midfielder Javed Khan, whose parents and grandparents were born in Fiji, was included in the training squad for the Pacific Games in January.
For more on Fiji football go to www.fijifootball.com.fj