The 'Girls' star wed her beau, CollegeHumor co-founder Ricky Van Even, at the exclusive Brush Creek Ranch in Saratoga, Wyoming, with guests including her co-star Lena Dunham, John Mayer and Katy Perry.
According to the New York Post newspaper, Oscar winner Tom oversaw proceedings at the sweet ceremony, while crooner John serenaded the couple.
The 27-year-old beauty was walked down the aisle by her father, NBC news presenter Brian Williams, and wore a stunning Oscar de la Renta gown.
Allison and Ricky, 34, kept their wedding top secret and even asked guests to fly into a different city, where they were picked up and taken to the luxury ranch location.
A source said: ''The wedding is being kept so secret that guests are being asked to fly to Denver, where they will be picked up and taken to the location, which is thought to be somewhere in Wyoming.
''The accommodations for all guests are being paid for and then many of the guests are flying to LA together for Sunday's Emmys.''
Allison previously admitted she had delayed her wedding to Ricky - to whom she has been engaged since February 2014 - due to the controversy surrounding her father Brian. The NBC news presenter was suspended earlier this year for embellishing details of his experiences while reporting on the front lines of the Iraq War and Hurricane Katrina.
In February, Allison, 27, said: ''This has been a really hard time. Obviously [it's] toughest on my dad, who is sort of bearing the full burden of the responsibility for it ... Current events started happening so that derailed [wedding plans] a little bit.''
His new album Underneath It All is out now.