Erin Cafaro, Elenaor Logan, Susan Francia, Caroline Lind and Caryn Davies were all back from the team that won the gold medal in Beijing in 2008 and they fulfilled their role as the heavy favorite in rowing's classic event.
Newcomers Ester Lofgren, Meghan Musnicki and Taylor Ritzel joined their more experienced teammates in holding off a late Canadian charge to win in 6:10.59
Canada took the silver medal in 6:12.06 and the bronze went to the Netherlands in 6:13.12.
It was the second rowing medal for the United States in the London Olympics, the other having come in the women's quadruple sculls on Wednesday.
Other gold medals won in Thursday's rowing finals were captured by the men's lightweight fours crew from South Africa and by Nathan Cohen and Joseph Sullivan of New Zealand in men's double sculls.
The South Africans pulled off a big upset, rallying over the last 500 meters to win gold in a thrilling, four-boat finish.
South Africa's margin of victory over Britain was just .25 of a second and Denmark was only .05 of a second farther back in third. Australia wound up fourth less than a second behind the Danes.
The New Zealand winners in double sculls are the current world champions and they had to come from behind late to defeat Italians Alessio Barton and Romano Battisti for the gold.
The bronze medal went to the veteran duo of Luka Spik and Iztok Cop of Slovenia, who have been rowing together for all of the 21st Century.
Cop, 40, first competed in the Barcelona Olympics in 1992, where he won a bronze medal in the pairs race, and this was likely his final competitive event.
There have been seven rowing finals contested and there are seven more to go, those to be held over the next two days on the course built alongside the River Thames near Eton College west of London.