Favourite Mo Farah was supposed to make it a straightforward duel with the American, but he failed to get into the medals and finished fourth. The Briton beat Lagat in a stirring 5 000 finish at the world outdoor championships in August.
And at 37, Lagat still has the finishing kick of runners half his age.
"It is not dying away," he said.
Lagat fully knew the Kenyans were going to make the race and refused to fall back and be surprised by a sudden breakaway. So when the final rush came, he was prepared.
"I am going to stay here because those guys are strong," Lagat said he thought to himself.
In the women's long jump, Brittney Reese jumped a championship record 7.23 metres on her last attempt to push American teammate Janay Deloach into the silver-medal position with 6.98. Shara Proctor took bronze with a British national record of 6.89 metres.
"I decided to get my confidence up and get the crowd involved," Reese said of her final attempt. "My coach told me to go and get it, and I went out and got it. I had to go out and bust the big one."
Lagat and Reese pushed the US gold medal haul to seven overall early in the final day. No other nation has more than one.
In the women's 800, 2008 Olympic champion Pamela Jelimo put years of physical problems behind her to take on the whole field from halfway and again became the front-runner she once was.
The Kenyan finished in a world leading 1 minute, 58.83 seconds to beat Nataliya Lupo of Ukraine and Erica Moore of the United States.