Tour convention dictates that the race leader is never challenged on the final stage, so Thomas only needs to reach the finish in Paris on Sunday to win.
Defending champion Chris Froome is also out of contention for the overall victory but leapfrogged Slovenian Primož Roglič, the victor of yesterday's final mountain stage, into third on the general classification standings after he crossed the line today in second.
Thomas may have known he had a buffer to play with, but he was not taking it easy as he was fastest through the first and second splits before grinding up the Col de Pinodieta and dropping back.
A former teammate of Thomas's and Froome's at Barloworld, when the Welshman was "maybe 10 kilogrammes heavier", Impey has seen him develop from an Olympic gold medal-winning pursuit specialist into a consistently strong performer on the road.
Thomas started the 105th edition of the race expected to provide support as Froome sought a record-equalling fifth Tour de France that would have allowed him to join previous greats like Jacques Anquetil, Eddy Merckx, Bernard Hinault and Miguel Indurain.
The Welshman's triumph is the sixth by a British rider since 2012 and saw him beat his team-mate and compatriot Chris Froome, the victor of four of the past five instalments of the three-week stage race. "I didn't know my wife was here either, which made it even worse".
But, when pushed into helping Thomas keep the yellow jersey in Sky hands after years of benefiting from the Welshman's help, Froome obliged on the bike - and when it came to handing out the plaudits afterwards.
"It's really nice Froomey committed fully to me and fair play", Thomas said.
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His eventual victory had been a mere formality after Saturday's individual time trial stage which left him with a one minute 51 second advantage over second-placed Tom Dumoulin of the Netherlands. "It's just bonkers. I'm going to try to enjoy today, and certainly tonight!"
Thomas first rode in the Tour in 2007, when he was 140th of the 141 finishers.
POOL New / Reuters Thomas drinks champagne with Great Britain Team Sky team principal, Sir Dave Brailsford, who rides in a vehicle.
Dumoulin may or may not be back for another tilt at the yellow jersey next year.
With his bonus seconds applied, Roglic moved into third place, 13 seconds ahead of Froome who slips to fourth, two minutes and 37 seconds behind Thomas.
France's Julian Alaphilippe and world champion Peter Sagan, of Slovakia, both finished within the time cut and are set to claim the polka dot jersey for the mountain classification and the green jersey for the points classification, respectively. "I'm surprising myself on this Tour but we're not in Paris yet".
"I'm really trying not to cry, it's unbelievable". He lost just 38 seconds to stay sixth overall but struggled on a climb on stage 19, losing more than 22 minutes to drop down the standings.