Taylor won the gold medal with a personal best of 47.25, leading teammates Kerron Clement and Bershawn Jackson across the line.
Until the American medals in that event, the bright spot for the United States in track and field had been the surprise gold medal won earlier in the day by Stephanie Brown-Trafton in the discus.
No American made the final in the long jump, Tyson Gay was non-competitive in the 100 and the United States will have no one in the final of the 1,500. The collection of medals in the 400 helped cure some of those ills.
Otherwise, the day's activities were highlighted by the first track gold medal ever won by a Kenyan woman, the first medal of any kind in these Games for Panama and the stunning departure of Chinese hero Liu Xiang.
Liu, defending champion in the 110-meter hurdles, fell to the ground after a false start in a preliminary heat. He limped out of the stadium with a strained Achilles tendon.
Pamela Jelimo won the women's 800 for Kenya. If she had not become the first woman from her country to win a track gold medal, Jeneth Busienei would have. Kenya finished 1-2 in the event.
Another Kenyan, Brimin Kiprop Kipruto, won the men's steeplechase and Panama's Irving Saladino Aranda captured the long jump.
Newly-crowned 100-meter champion Usain Bolt easily made it through two rounds of 200 qualifying and defending 400 champion Jeremy Warnier won his first-round heat as well.