Julian Fellowes's period drama's first episode had a consolidated audience of 11.428 million (39% share), it has been revealed.
BBC One's Call The Midwife was the previous biggest with 11.408% (a 35% share).
Downton added 2.428 million viewers with seven days of recorded viewing added on top.
However, the stats don't include iPlayer viewers, which may have tipped Midwife ahead.
Last night's Downton Abbey collected an audience of 8.1 million in the overnight ratings. The show won the 9pm hour, but it was one million viewers down from the equivalent episode last autumn.
Downton Abbey continues on Sunday nights on ITV1.
[Downton Abbey Episode 2 - Photo Gallery]