In a dramatic game, Cesar Azpilicueta and Michy Batshuayi put Chelsea 3-1 up, Daryl Janmaat's solo goal and a strike from Stefano Okaka pulled Watford level, but Blues substitute Cesc Fabregas hit a late victor while the Hornets had Sebastian Prodl sent off in stoppage time.
Chelsea were in party mode at Stamford Bridge on Monday night as they celebrated their Premier League title in a game that was more like a John Terry testimonial than a competitive league match.
Terry, who is leaving this season after 22 years with the club, hooked home a loose ball from a Willian corner in the 22nd minute.
Conte can do no wrong and he was being cheered at Stamford Bridge within seconds of appearing in his technical area after winning the Premier League at the first time of asking.
John Terry also scored in a rare start but two defensive lapses from the 22-year Chelsea man almost allowed Watford to salvage a draw before Fabregas' late victor.
The 26-year-old has been remarkably consistent in that role and produced yet another impressive performance last night as Chelsea pushed themselves over the finishing line with a bitter-fought, tetchy and dramatic 1-0 win over West Bromwich Albion at the Hawthorns.
Two minutes later, Terry gifted Watford a rare away goal scored by Etienne Capoue, but Cesar Azpilicueta's crisp drive restored Chelsea's lead.
Chelsea manager Antonio Conte, who made nine changes to his starting XI from the side that beat West Brom to clinch the title, said the club was now aiming to beat Sunderland in Sunday's final round to reach a record mark of 30 wins for the season.
Sebastian Prodl was sent off for a second bookable offence in the second minute of stoppage time as Watford finished the contest with 10 men in a bad-tempered conclusion.
Asked if Terry should retire, Conte said: "No".
Fireworks lit up the sky in south-west London as Mason blew the final whistle while blue-and-white streamers rained on the pitch.
A 16th will be sought in the May 27 FA Cup final with Arsenal, when Antonio Conte will bid to end his first season as head coach with the double. "I must be pleased because the answer was very good", Conte told the BBC.