Bill Mastro was born on 6 December 1952 in Summit, New Jersey, USA. He has been married to Mary Lou Bonnamy since 11 September 1982. They have three children.
His father - a shoe store owner and former member of the Bernardsville (NJ) Board of Education - petitioned to have his and his family's last names changed legally to "Mastro"; the petition was granted by the Somerset County (NJ) Court on November 15, 1957.