The season two closer, which aired on Sunday night, was watched by 5.4m viewers.
The ratings, which do not include people who watched online or on DVR, are the highest for a PBS show in three years.
Overall, Downton Abbey also reached an audience of more than double the network's primetime average and improved its ratings from season one by nearly 30%.
"It is now clear that the awards success at the Golden Globes and the Emmys has brought Downton Abbey to the attention of a far bigger audience in the US than a British show would normally achieve," said executive producer Gareth Neame of Carnival Films.
"US audiences have embraced Downton in the same way that they have in the UK and around the world."
Bones star David Boreanaz recently tweeted that he would like a role on Downton Abbey after watching the season two finale.
Shirley MacLaine has already signed up to appear in the third season, while War Horse actor Matt Milne has joined the cast as a new footman.