D'Agosto was born in Omaha, Nebraska, the son of Deanna Rae (née Vellinga) and Alan D'Agosto, Omaha's major Arby's restaurant franchisee. His mother is from Sioux City, Iowa, while his father is from Omaha.
He has two older brothers, Andrew and Patrick, and two younger sisters, Julie and Katie.
D'Agosto was raised Catholic and selected Genesius of Rome, the patron saint of actors, as his confirmation name. D'Agosto said that he wanted to be a priest when he was young, then when he was 10 years old he started getting into improv.
D'Agosto was very involved in drama and theatre growing up, acting in many different plays and musicals. He participated in competitive forensics speech tournaments during sixth, seventh, and eighth grades. He graduated from Creighton Preparatory School in 1998 and attended Marquette University, where he graduated cum laude in 2002 with a double major in history and theatre.
