Slattery was born in Boston, Massachusetts, to Joan (née Mulhern), a retired accountant, and John "Jack" Slattery, a leather merchant. He is one of six children. As a young boy, he dreamed of being a baseball player. Slattery is of Irish descent and was raised Catholic. He attended high school at Saint Sebastian's School in Newton, Massachusetts (since relocated to Needham, Massachusetts), and received a Bachelor of Fine Arts from The Catholic University of America in 1984.