The 22-year-old joined the early break after a frenetic start from Saint-Paul-Trois-Châteaux in windy conditions saw 14 riders attack including Julian Alaphilippe (Deceuninck-QuickStep), Edward Theuns (Trek-Segafredo) and Stefan Kung (Groupama-FDJ), with most sitting back in the peloton after yesterday's gruelling mountain stage.
Sweeny then attacked with 40 kilometres to go to form a lead group of four as Kung, Imanol Erviti (Movistar) and Nils Politt (BORA-Hasngrohe) followed, before going again on the uncategorised climb 14 kilometres from the finish, dropping Kung in the process to make a three-man contest for the win.
Politt ultimately proved the strongest of the contending trio, racing away from Sweeney and Erviti to stretch out a big lead in the final kilometres and solo to just the second professional victory of his career. Erviti outsprinted Sweeney in a close race for second with the Queenslander taking third as a reward for his strong efforts on the day.
Sweeney admitted his ride wasn't a perfect one, but was still satisfied with the performance as he beat out a strong field of competitors on the stage in his debut Tour de France.
“I think I made a few neo-pro mistakes there attacking and showing my strength in the final," he said.
"But the guys that I was in the break with and the guys that I beat today, I’m pretty happy with that. I’m not known for my sprints so I can’t be too disappointed.
"I saw Stefan (Kung) was suffering on the climb so I gave it my best and tried to drop everyone, but they managed to get back.
With more pressure to perform placed on his and the rest of the Lotto Soudal riders shoulders due to Caleb Ewan's early injury, Sweeney felt today's ride showcased his talent and willingness to execute the team's newfound aggressive strategy.
"I feel like I’m finally reaching my potential with Lotto," he said.
"They’ve given me a lot of responsibility this year and I think I’ve showed that I’m worthy of it, so I’m really happy I can do a good job for the team.
"It’s been a hard tour for us and we said that we’d race really aggressively, I think I showed that today."
The Tour de France will continue with Stage 13, a 220 kilometre stage from Nîmes to the medieval town of Carcassonne. Watch from 2030 AEST on SBS, SBS OnDemand, with the race coverage starting on the SKODA Tour Tracker from 1955 AEST.