The Mitchelton-Scott rider looked around half in surprise on the finish line to confirm his victory in the three-up dash to the line at the end of a mountainous day in the saddle.
To his delight, he passed over the line ahead of Pello Bilbao (Astana) and Gregor Muhlberger (BORA-hansgrohe) claiming his first Tour de France stage win.
"I wasn't confident beating either of them, I didn't know how fast they were," said Yates. "I just knew from the director in the car that I needed to be very quick round the last corner, so I made sure to do that and luckily I held onto it."
Simon is playing right-hand man to twin brother Adam in the pointier parts of the race, who is maintaining a strong general classification ride in seventh overall currently.
"I'd been conserving energy until I got here in the mountains, said Simon Yates. "It was my first chance to try something. Normally, I'd be back in the peloton helping Adam but today I had my own chance. So I grabbed it with both hands."
"I'm very proud. We're having a fantastic Tour and long may it continue."
[tdf widget="stagewinners" stage="12"]
After 30 kilometres of fighting to establish the breakaway, the peloton allowed a massive group of 40 riders to get up the road and establish a gap that increased gradually to a maximum seven minute advantage.
Deceuninck-Quickstep took the responsibility of driving the pace on the flat into the first of two category 1 climbs on the stage, with the peloton seemingly keen to take an 'off' day before the individual time trial on Stage 13.
Simon Clarke (EF Education First) attacked over the top of the climb of the Col de Peyresourde, allowed away after the others in the breakaway finished battling it out for the mountain points. The Australian built his gap gradually to a lead of one and a half minutes as the large chasing group conserved energy behind.
So slow was the pace of the chasers a number of attacks tried to bridge to Clarke, with Matteo Trentin's (Mitchelton-Scott) surge sparking a number of aggressors clear from the break.
Behind the peloton, Australian Rohan Dennis (Bahrain Merida) strangely stopped by the roadside, showing up on the finish line with personal agent Andrew McQuaid flanking him, his team apparently unaware he abandoned, tweeting they were investigating the situation.
Once on the climb of the Hourquette d'Ancizan, Clarke was reeled in, first by Trentin, then by his team mate Yates followed by Muhlberger with Bilbao nearly making contact on a few occasions.
As Yates and Muhlberger crested the top of the category 1 climb together, Bilbao joined them on the descent, the trio working well together on the 30km plunge to the finish, only looking intently at each other in the final kilometres.
In the end Yates was rewarded for jumping first, fastest through the final corner and able to box out Muhlberger until the finish line.
The peloton had a relatively sedate day for a mountain stage arriving as a large group with few riders dropped and no significant change in the general classification battle.
[tdf widget="tourleaders" stage="12"]
