A South Boston native, born in the Hub on Oct. 17, 1960, a son of the late John and Catherine (Connell) Costello, he is the third of six siblings: an older brother, Patrick, and an older sister, Kathy; two younger sisters, Jayne and Lisa; and a younger brother, Seán.

A son of St. Brigid of Kildare Parish, he attended the parish elementary schools prior to entering Don Bosco Technical High School, Boston. He was granted a bachelor of science degree in civil engineering from Northeastern University and a master's degree in business administration from Suffolk University.

He and his wife, Carol (McGlone) Costello, are parishioners at the Resurrection of Our Lord and Savior Jesus Christ Parish, Hingham, and are the proud parents of three children: Caitlin, who lives in the old neighborhood in Southie, and John and Margaret, who live in Hingham.

He is presently the city engineer in Quincy and was previously employed for three decades in several private civil engineering consultants, including Design Consultants, Somerville; Polaris Consultants and Environmental Partners, both in Quincy; SEA Consultants, Cambridge; and Geotechnical Engineers in Winchester.

During his deacon formation program, he had pastoral assignments at Resurrection, Hingham, and at Beth Israel Milton Hospital, Milton.