The 24-year-old American made a 26-foot putt for birdie on his last hole as he produced the day's best score and won at 18 under 262, sealing his spot in the FedEx Cup playoffs.
Thorbjornsen had rolled into the Motor City on the playoffs brink at No 69 in the standings. The top 70 advance to the postseason after next week's Wyndham Championship.
World No.15 Xander Schauffele surged home with a 65 to claim second place with overnight leader Davis Riley (70) settling for third at 15 under and Denmark's Rasmus Hojgaard (68) fourth at 14 under.
Hideki Matsuyama (64), Matt Wallace (67), Kristoffer Ventura (68) were another stroke back, tied for fifth.
Karl Vilips, the lone Australian to make the cut, shot a 75 to place tied 65th at one under.
Thorbjornsen started the final round three shots behind Riley, whose rough start opened opportunities for the field. He took advantage, surging into the lead at No. 14 with his third of four birdies in a six-hole stretch and closing with a clutch, curling putt that went in the right side of the cup.
He's the eighth first-time winner this season on the PGA Tour and the second straight, following Jackson Koivun's victory last week at the 3M Open.
"It was such a battle, playing with some great players," Thorbjornsen said. "Everyone was in the mix after nine holes."
Thorbjornsen chipped in from 35 feet for birdie at No. 12 on a day when he accurately aimed for pins that left him with some short birdie putts before his long one on his 72nd hole.
He had a bogey-free round, an accomplishment many other players on the leaderboard were unable to pull off on a day that started with rain and became windy when the clouds cleared.
Riley started with a one-shot lead and quickly lost it — and couldn't find his ball in fescue at No. 4 after about a dozen people helped him try — with three bogeys through six holes.
He rallied with three birdies in a seven-hole stretch before giving back a stroke with a bogey at No.17, taking away his chance to possibly force the tournament into a playoff.
The eighth and final Rocket Classic drew its strongest field and biggest crowds.
The PGA Tour is moving to a new model for 2028, effectively creating two tours and Rocket Mortgage decided not to exercise its option to host another tournament next year.
The Detroit-based company isn't interested in continuing to spend $US15 million a year to be part of the second tier in the new era and didn't want to double the costs to be part of the elite tier.
- with AAP
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