He graduated from Saint Michael's College in Burlington in 1968;[3] he went on to earn his Juris Doctor degree from Boston University Law School in 1974.
From 1968 through 1971, Costello was an officer (First Lieutenant) in the United States Marine Corps and served in the Vietnam War.
Costello served in the period between his graduation from Saint Michael's and his enrollment at Boston University Law School.
On September 9, 2008 Costello faced off against Nate Freeman, a local businessman, in the primary.
Amid a low turnout Costello won with 61% of the vote but lost to incumbent Republican Brian Dubie in the general election.