Using every inch of his 2.03 metre (6-foot-8) frame, Grevers stretched his long arms out to win in a time of 52.16 seconds, a new Olympic record and just 0.22 outside the world record set by Aaron Peirsol.
Grevers is the latest in a long line of great American backstrokers, but like his predecessors, he had to bide his time before he took the top spot.
He served as the understudy to Peirsol, winning silver behind in Beijing and he hoped to follow the tradition with Thoman, whose grandfather once held the world record, finishing in 52.92.
Americans have finished 1-2 in nine of the 23 Olympics 100 backstroke finals.
"I saw I won and I was celebrating and Nick hit me and I saw second and it was a whole new celebration and it felt even better," said Grevers. "I got second to Aaron in 2008...it's the great cycle of swimming."
“I went out and gave everything,” said Tancock, who has won a medal at every major international tournament except the Olympics.
“It didn’t come off but Rio is only four years off. I love what I do – swimming is a great sport, it gives you so many friends and keeps you fit.
“This crowd is amazing and it’s a dream to swim in front of so many supporters. I came here today and gave it my best.
“I was walking through the call room and it said ‘inspire a generation’. I hope that’s what Team GB are doing and I hope we inspire a future generation of swimmers and Olympians.
“It was fantastic and the home support is brilliant. I can’t ask for more, and it makes us swim faster, and I can’t wait for more performance later in the week.”
With Japan's Ryosuke Irie taking the bronze, there was disappointment for world champion Camille Lacourt, who had high hopes of a medal after missing out on Beijing due to a back injury.
Just minutes after French compatriot Yannick Agnel won gold in the 200 metre freestyle, and with French President Francois Hollande watching from the stands, Lacourt could only manage fourth.
"I will try to pick myself up," a visibly shaken Lacourt said. "Do what I've got to do. My only regret is that we can't find what we've got to do to make it work."
- Aaron Peirsol