Allison Williams is the daughter of Jane Gillan Stoddard and NBC Nightly News anchor Brian Williams. Williams was raised in New Canaan, Connecticut. She attended Yale University and graduated in 2010. While at Yale, Williams was a member of the improv comedy troupe Just Add Water for four years, as well as being a part of the youtube web series College Musical, and was inducted into St. Elmo.
In 2010, Allison Williams performed a mashup of "Nature Boy" set to RJD2's "A Beautiful Mine", the theme song for the television series Mad Men. The YouTube video of the performance received widespread praise on the Internet, and convinced Judd Apatow that she should be given a part in the HBO TV series Girls.