Sherlock and Doctor Who were among the favourites to win prizes at the London ceremony after garnering a total of five nominations between them, but it was Julian Fellowes' popular series, starring Dame Maggie Smith, Hugh Bonneville and Michelle Dockery, which landed the top honour.
Benedict Cumberbatch's portrayal of the fictional detective and Matt Smith's role as the Time Lord were also snubbed in the Best Male Performance in a Drama category - Colin Morgan claimed that title for his axed BBC series Merlin.
Former Doctor Who star Karen Gillen also missed out on an award - she lost out to Call the Midwife's Miranda Hart in the Best Female Performance category.
Meanwhile, long-running soap opera Coronation Street was a double winner at the event - it was named Best Serial Drama and Best Serial Drama Performance went to Alan Halsall.
Sir David Attenborough's Frozen Planet was named Best Documentary Series and Best Situation Comedy was handed to Mrs Brown's Boys.
Other winners at the annual TV prizegiving included Eastenders' David Witts (Best Newcomer), I'm a Celebrity... Get Me Out of Here! (Best Entertainment Programme), This Morning (Best Daytime Programme), QI (Best Comedy Panel Show) and Strictly Come Dancing (Best Talent Show).
Meanwhile, Anthony McPartlin and Declan Donnelly were named Best Entertainment Presenters for the 12th year in a row.