It denoted the first triumph for Lowry since winning the European Visit's Portugal Bosses in 2012 and his lady win on US soil.
The 28-year-old Lowry sank four birdies including a 10-foot putt on the 72nd gap to complete with a 11-under 269 aggregate in the last tune-up before the PGA Title at Shrieking Straits.
Two-time significant victor Watson coordinated Lowry's 66 on Sunday however completed two strokes back with a nine-under 271.
It was another baffling completion at the Firestone Nation Club for American Jim Furyk who completed in the main 10 for the tenth time however is as yet looking for his first WGC-Bridgestone triumph.
Furyk let go a two-more than 72 and tied Britain's Justin Rose (72) for third place, four strokes back of Lowry.
Ruling US Transparent champion Jordan Spieth shut with his best round of the competition by shooting a four-under 66.
Spieth came in six birdies Sunday and said he despite the fact that he is not playing his best golf at this time he is prepared for the test of attempting to win his third major of the season in Wisconsin.
Lowry started his last cycle two shots off the pace set by third round pioneers Rose and Furyk.
He wiped out that crevice with birdies on the No. 2 and No. 8 and after that took a two-shot lead with another birdie after the turn at No. 10.
Lowry clutched his two shot lead with a splendid standard recovery from 20 feet on the fourteenth subsequent to discovering a fairway shelter with his drive.
Lowry moved to 11-under standard when he sank his birdie putt at the last to win his first WGC title.
Lowry takes after Northern Irishman Rory McIlroy in winning the WGC-Bridgestone however appeared to be perplexed around two Ireland players retreating to-back.
Robert Streb (66) came in fifth at 274. Rivulets Koepka (69), Sweden's Henrik Stenson (70) and David Lingmerth (68), and New Zealand's Danny Lee (68) tied for 6th at 275.
Spieth tied Rickie Fowler (67) for tenth spot. English Open champion Zach Johnson shut with a 71 and tied for 33rd spot at 282. (AFP)