John was born the fourth of seven children of Philip and Loretta Keenan. His father worked in the mailing room of the Record American, and then the Boston Herald, while his mother worked at Filene’s department store and then CNA Insurance. John was raised in Quincy and attended the Quincy Public Schools, graduating in 1982 from North Quincy High School. He went to Harvard University, earning his way through by working in the athletic department equipment room. He also worked the Saturday overnight shift at the Herald. John graduated with honors from Harvard in 1986, with a government concentration. Three years later, he graduated from Suffolk University Law School. A lifelong learner, in May 2019 John graduated from the Harvard Kennedy School with a Masters in Public Administration.