Mantello started his New York stage career directing Imagining Brad at the Players Theatre in 1990. He won the 2003 Best Director of a Play Tony Award for Take Me Out. He also won the 2004 Best Director of a Musical Tony for Assassins.
Acting credits include Broadway's Angels In America and The Normal Heart, which both earned him Tony nominations
Broadway directing credits include What's Wrong With This Picture?; Love! Valour! Compassion!; Proposals; Design for Living; An Evening with Mario Cantone; Frankie and Johnny in the Clair de Lune; Wicked; Laugh Whore; Glengarry Glen Ross; The Odd Couple; Three Days of Rain; The Ritz; November; Pal Joey; 9 To 5; Other Desert Cities; The Other Place; I'll Eat You Last: A Chat With Sue Mengers; Casa Valentina; The Last Ship; Airline Highway and An Act of God.