Guaraldi was born in San Francisco's North Beach area, a place that became very important to his blossoming musical career. His last name changed to "Guaraldi" after his mother, Carmella (née Marcellino; 1908-1999), divorced his biological father (whose last name was Dellaglio) and married Tony Guaraldi, who adopted the boy. Growing up, Guaraldi was influended by both of his maternal uncles, Joe and Maurice "Muzzy" Marcellino, both of whom headed jazz big bands in San Francisco for many years. He graduated from Lincoln High School, briefly attended San Francisco State College, and served in the United States Army as a cook in the Korean War.