Benjamin timed 46.46 seconds as defending champion Warholm tied up badly towards the end of the race, claiming silver in 47.06sec.
Brazil's Alison Dos Santos took bronze, as he had done at the Covid-delayed Tokyo Games, in 47.26sec.
"I can't believe I actually finally did it," said Benjamin, who won silver in Tokyo as well as two world silvers and a bronze.
"It just happened at the right moment. My family is here, my friends are here. To do it in front of them means so much to me. I'm just so excited."
Benjamin added: "I probably knew I had won five metres before the line.
"I thought I'd get dipped, but no one was there so I thought 'wait a minute, hold on, I'm actually doing this'. And I did it. It's just a great feeling."
Warholm smashed the world record to claim gold in Tokyo, clocking a stunning 45.94 seconds to win ahead of Benjamin and Dos Santos.
But there was no repeat at a packed 69,000-capacity Stade de France.
Warholm had come into the Paris Games under pressure, having lost to both Dos Santos and Benjamin this season, at Diamond League meets in Oslo and Monaco respectively.
The Norwegian, a three-time world champion, bolted out of his blocks in lane seven in his typically aggressive manner.
Benjamin was outside him in lane eight and the Norwegian was quickly a length ahead of the American.
But coming around the final bend into the home straight, Benjamin kept his rhythm and took the lead with three hurdles left to clear.
Warholm stuttered and it was all advantage Benjamin, the 27-year-old American turning the screw and powering home for a long-awaited victory over the Norwegian in a global championships.
"To do it here means a lot," Benjamin said.
"I've been doing it all season. To get it done it here when it really matters, when everyone said I couldn't get it done, it means so much to me.
"This one, they can't take away from me, at all. Mostly for me, this is about proving myself right.
"It does suck to see people doubting your abilities, not knowing the full story of what's going on. I've been battling injury, there's a lot of stuff that happens off the track that nobody really sees.
"Just to come out here and have everything smooth means a lot."
