He said the victory was particularly special given his grandfather, who played in the 1960 US PGA Championship at Firestone, was in the gallery.
He is an eight-time victor at Firestone Country Club but has not won anywhere since his 2013 victory on the course, which is hosting the event for a final time before it moves to Memphis next year.
The world number three, who will start the defence of his US PGA Championship next Thursday, birdied the second hole but dropped back to 14 under with a bogey at the par-four sixth.
Kyle Stanley won the race for second after a 68 lifted him to 11 under, but Thomas was relatively untroubled down the stretch and was content to grind out the pars with none of his rivals able to maintain a significant challenge.
McIlroy made up three shots in the final round in 2014 when he rallied to beat Sergio Garcia at Firestone. Dustin Johnson and Thorbjørn Olesen finished tied for third at 10-under.
U.S. Open champion Brooks Koepka had a 67 to finish fifth.
"It feels awesome", Thomas, who has now won eight times since October 2016, told Sky Sports.
Thomas was far from his best as he played the opening 12 holes in level par and both Stanley and Jason Day eventually closed to within two shots, only to each promptly bogey the next two holes.More news: Kerala: After Nipah, Outbreak of West Nile Viral Infection Suspected in Kozhikode
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Fleetwood didn't put a foot wrong at Firestone Country Club, where he said his early tee time was a big advantage on Firestone's fast greens.
Thomas' ninth title on the PGA Tour will bank him a cool $US1.7 million cheque and will move him up one spot on the world rankings.
"I'm excited for Sunday to see how everything holds up", said McIlroy. He finished with a bogey on 18.
Golf-loving Wayne long saw this coming.
She was at 13 under overall, one stroke ahead of playing partner Georgina Hall of England, whose scrambling through an erratic round kept her in contention for her first major.
Hall is the first English major victor since Karen Stupples won this event in 2004, and the fourth overall along with Laura Davies and Alison Nicholas.
She followed Stupples and Catriona Matthew - in 2009 at Lytham - as the only British winners of the Women's British Open since it achieved major status in 2001. "I think I got away with one bad tee shot where it bounced out of the trees, but apart from that played lovely, putted really nice and just picked them off".
Pornanong has been singing Thai songs in her head to calm herself on a links course she admits to struggling on.