Harman, a PGA Tour stalwart and an avid hunter back home in Georgia, offered hope for those wanting a thrilling conclusion to the 151st championship with a few early errors.
But the diminutive left-hander proved elusive prey for his bedraggled pursuers as he turned the final day into a procession, calmly plotting his way to the Claret Jug with a one-under closing round of 70 for a 13-under total of 271.
"I got to thank my wife, my family back home. I sure miss them.
"I can't wait to get back to them tomorrow. First I'm going to have me a couple of pints out of this here trophy, I believe," the no-nonsense Harman said as rain dripped off his shaven head at the presentation.
Rarely has an Open been concluded in such appalling weather conditions but the diminutive Harman did not care one iota as he spurned the offer of an umbrella to be presented with the Claret Jug, rain streaming off his shaven head.
"I'm looking at the forecast and then get out there and it's like Armageddon, it was really tough," Harman, who will celebrate back home by taking his new tractor for a ride, told reporters.
"I haven't historically done very good in the rain. It's just always bugged me but I was really proud of the way that I struck the ball today.
"Hats off to the fans. Damn sure I wouldn't have been out watching golf today."
Few of the thousands huddled under a sea of umbrellas would have tipped world number 26 Harman to win before the tournament.
But he proved that the old adage 'drives are for show and putts are for dough' still applies as his magic touch on the greens kept him out of reach.
Spain's Jon Rahm, Austria's Sepp Straka, South Korea's Tom Kim and Australian Jason Day were tied for a distant second place with pre-tournament favourite Rory McIlroy and Emiliano Grillo a further shot back.
IN CONTROL
Harman had seized control of the tournament with a sensational second-round 65 on Friday and never loosened his grip. He began Sunday's climax five shots clear of compatriot Cameron Young and six ahead of Rahm who had roared into contention with a course record 63 on Saturday.
It needed someone to shoot very low or a Harman collapse to make things interesting and when the American bogeyed the second hole and the fifth, after driving his ball into a gorse bush, Masters champion Rahm briefly closed to within three strokes.
Harman's previous best performance at a major was in the 2017 US Open when he led by one stroke after 54 holes only to be overhauled by fellow American Brooks Koepka.
But this time, with the largest three-round Open lead since Rory McIlroy won on the same course in 2014, he responded to every setback in style with his trusty putter again proving to be his weapon of choice.
Birdies at the sixth and seventh eased any nerves and Harman reached the turn with his five-shot lead intact as Austrian Straka and Kim emerged as his unlikely challengers.
Incessant rain dampened the spirits of those trying to reel him in, although four-time major winner McIlroy had threatened to spice things up with three birdies in his first five holes.
Once again, however, the Northern Irishman could not sustain his charge as his near 10-year wait for a fifth major goes on.
Likewise, Young, Rahm, Day and England's Tommy Fleetwood were all unable to hole the putts that would have at least asked some questions of Harman's nerve.
SOGGY END
McIlroy's fire was doused in the deluge although his 68 was his best round of the week. Local favourite Fleetwood looked like a drowned rat as his challenge came to a soggy end with a triple-bogey after finding the dunes at the back of the 17th.
Harman's third bogey of the day, at the 13th, opened the door ever so slightly and Straka momentarily got within three strokes when he holed his fifth birdie of the day at 16.
But rock-steady Harman responded to sink a curling birdie putt at the 14th and when he rolled in another at the 15th the Claret Jug was as good as in the bag.
The treacherous par-three 17th and long 18th could have been nerve-racking for a player who had not tasted a victory of any sort since 2017. But not even a Merseyside monsoon could rain on Harman's parade as he enjoyed a victory march down the last.
He was denied a fourth successive round in the 60s but after rolling in his par putt his poker face finally cracked into a smile as he bumped fists and embraced his caddy Scott Tway.
Shubhankar Sharma tied for eighth place to post the best finish by an Indian at the British Open - one of only 22 players to finish an attritional Open under par.
