His father was half English and half Italian. From his mother's side, he is of Irish, Dutch, Scottish and Cherokee (Native American) descent.
Armstrong was born on February 17, 1972, in Oakland, California, and raised in nearby Rodeo, the youngest of six children of Andrew "Andy" Marsicano Armstrong and Ollie Jackson. His father worked as a jazz musician and truck driver for Safeway Inc. to support his family. He died of esophageal cancer in September 1982, when Armstrong was 10. He has five older siblings: David, Alan, Marci, Hollie, and Anna. His mother worked as a waitress at Rod's Hickory Pit restaurant in El Cerrito, where Armstrong and Dirnt played their first gig in 1987. Armstrong's great-great-grandparents Pietro Marsicano and Teresa Nigro were Italian immigrants from Viggiano, Basilicata who moved to Boston, Massachusetts before arrived to Berkeley, California in 1869. For this, he received the honorary citizenship of Viggiano in June 2018 by mayor Amedeo Cicala.