The 25-year-old Australian, in his first Tour, came around Groenewegen after the Dutchman launched his sprint with 200 metres of the 167km stage 11 from Albi remaining and took it by inches.
Ewan, who has stage wins in the Giro d’Italia and La Vuelta to his name, had made no secret of the fact a Tour victory was what he craved the most.
“I can’t believe it,” he said. “I’ve been close in the last four sprints and my team never lost faith in me and I never lost faith in my sprinting. I knew if everything came together I could be fastest on the day and today I showed that.
“Since childhood there is no other race I’ve dreamt of winning. Watching in Australia, the Tour seems so far away – I can’t believe I’m even here but to win a stage is a dream come true.”
Yellow jersey holder Julian Alaphilippe and defending champion Geraint Thomas finished in the pack to ensure there was no change at the top of the general classification, in which Alaphilippe leads Thomas by 72 seconds as the race hits the Pyrenees today.
Thomas’ Ineos team-mate Egan Bernal finished 13th on to stay a further four seconds behind Thomas in third place.
After Tuesday’s rest day and Monday’s chaotic finish in crosswinds, Wednesday’s stage was a much more sedate affair as the peloton followed the Tarn west out of Albi.
A four-man breakaway including serial escapee Stephane Rossetto of Cofidis was kept on a short leash, with their advantage never quite reaching three minutes, a gap which began to tumble as Toulouse came into view.
Threatened crosswinds never materialised in the finale, although there was a little late drama as a crash held up Movistar’s Nairo Quintana and Trek-Segafredo’s Richie Porte, who needed five kilometres to be paced back onto the pack.
However, Porte’s team-mate Giulio Ciccone – who wore the yellow jersey for two days after finishing second on stage six to La Planche des Belles Filles – was not so lucky as he limped home with injuries that saw him lose his 10th place in the general classification.
Wanty Gobert’s Aime De Gendt went solo from the breakaway with 10 kilometres left, pulling more than 40 seconds clear, but he was reeled in as the sprint trains moved up.
Mike Teunissen, winner of the opening stage in Brussels, led out his Jumbo-Visma team-mate Groenewegen but after he pulled off, Ewan had the power to come around the Dutchman in the nick of time.
Alaphilippe’s Deceuninck-Quick Step team-mate Elia Viviani took third with Bora-Hansgrohe’s Peter Sagan fourth in the green jersey.
The next moves in the battle for yellow could come on today’s 209.5km stage 12 from Toulouse to Bagneres-de-Bigorre via climbs of the Peyresourde and the Hourquette a’Ancizan.