The colt - sent off the 9/4F - swept past early pacesetter We the People in the stretch and held off barn-mate Nest for a comfortable three-length success, with Skippylongstocking for trainer Saffie Joseph Jr and Manuel Franco a distant third.
The three-year-old's win came off the back of a Stakes success at Aqueduct on April 9 and handed Long Island resident Pletcher his fourth showpiece Belmont victory.
The trainer lifted the lid on the advice he gave Ortiz Jr knowing that the horse had the late gears to close out the win.
"Be patient," Pletcher told reporters. "I think you have the best last quarter of any horse in the race."
"To be honest with you, we were a little confident going into the race today," Donegal Racing CEO and co-owner, Jerry Crawford, said.
"When he turned for home, I was like, forget about it. I know Todd thought he could get a strong last quarter mile, and he surely did."
"This is New York's biggest race and to win it here, with my family and friends and 70 people here, this will be a big winner's circle," co-owner Mike Repole added.
Rich Strike, a shock 80/1 Kentucky Derby winner, finished a disappointing sixth having been left out of the Preakness with one eye on Belmont.
Trainer Eric Reed said the team had suggested jockey Sonny Leon should try get Rich Strike to push from the outside, but the horse kept wanting to get on the inside and never landed a blow.
"I think we just made a tactical error," Reed said.
Mo Donegal finished the one-and-a-half-mile distance in 2 minutes, 28:28 seconds in front of a 46,103 crowd and his victory meant the Triple Crown contests were won by three different horses for the fourth straight year - a first for the sport since 1926-29.