The current world champion was more than a half second in front of his nearest challenger, an unusually large margin for the race, and cemented his reputation as the best in the event.
American Jennifer Suhr closed the evening with a gold medal in the pole vault, the only one won by a United States athlete anywhere at the Olympics Monday.
It was the American absence in the 400, however, that stood out.
Only once before in the history of the Games had a men's 400-meter final been run without a runner from the United States taking part. That came in 1980, when no Americans took part in anything at the Olympics because of the boycott of the Moscow Games.
Americans swept the medals in the 400 in Beijing four years ago, did the same thing four years before that in Athens and won the top two places in Sydney in 2000. Since the 1980 boycott, the event had been won by an American in every Olympics until Monday night.
Even if the three U.S. runners who came to London to participate in the event had been in it, however, the winner would likely have been the same.
James ran the distance in 43.74 and in so doing won the first Olympic medal ever for Grenada.
Luguelin Santos of the Dominican Republic finished second in 2:44.46, followed by Lalonde Gordon of Trinidad and Tobago in 3:44.52.
Suhr cleared 15 feet, 7 inches to win the pole vault. Cuba's Yarisley Silva also made it over that height, but she had more misses during the competition than did Suhr and had to settle for the silver medal. The bronze went to Elena Isinbaeva of Russia.
The evening of track and field finals began with Nadzeya Ostapchuk of Belarus winning the women's shot put, after which Felix Sanchez of the Dominican Republic won the 400-meter hurdles eight years after his first gold medal in the event. Michael Tinsley of the United States finished second to Sanchez for the only medal other than that of Suhr's won by an American in track and field Monday.
Russia's Yuliya Zaripova followed all that by running away down the stretch to capture the 3,000-meter steeplechase.
In qualifying for the women's 200-meter dash, both 400-meter winner Sanya Richards-Ross of the United States and 100-meter winner Shelly-Ann Fraser-Pryce of Jamaica made it safely through.
So did Jamaica's Veronica Campbell-Brown, who has won the event in each of the last two Olympics. Richards-Ross had the fastest time in the qualifying heats -- a 22.48.