Gilmore was cast as Bobby Draper in Mad Men in 2009. He was the third actor to play the character, after Maxwell Huckabee and Aaron Hart, since 2007 when the character was originated.
In 2011, Jared left Mad Men and was cast in Once Upon a Time as Henry Mills, the "biological son of Emma Swan (Jennifer Morrison's character) and Neal. He is the only resident of Storybrooke who is not under the spell of the Evil Queen (Lana Parrilla)." Series creators Adam Horowitz and Eddy Kitsis stated of the role, "One of our emotional centers was an 11-year-old boy who had to be precocious and vulnerable at the same time." Kitsis believed that Gilmore "naturally brought [these characteristics] out... We just knew he had to be our Henry!" Gilmore commented, "I relate to Henry, because I'm 11 and also have a very good imagination. I enjoy making up and playing games in worlds with alternate realities myself."
At the 33rd Young Artist Awards, he won the Award for Best Performance in a TV Series – Leading Young Actor. The following year, at the 34th Young Artist Awards, he was nominated for Best Performance in a TV Series – Leading Young Actor. He was also included among the cast of Mad Men when it won the Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series. in 2010 for season 3.