Born in Brooklyn, New York, Blumenthal is a graduate of Harvard College, where he was editorial chairman of The Harvard Crimson. He studied for a year at Cambridge University in England before attending Yale Law School, where he was editor-in-chief of the Yale Law Journal. While at Yale, he was classmates with future President Bill Clinton and future Secretary of State Hillary Rodham Clinton. From 1970 to 1976 Blumenthal served in the United States Marine Corps Reserve, where he earned the rank of sergeant.
After college Blumenthal served as administrative assistant and law clerk for several Washington figures. From 1977 to 1981 he was United States Attorney for the District of Connecticut. In the early 1980s he worked in private law, including volunteer counsel for the NAACP Legal Defense Fund. He served in the Connecticut House of Representatives from 1984 to 1987, when he was elected to the Connecticut Senate. He was elected state Attorney General in 1990, and served for twenty years. During this period he was frequently speculated as a contender for Governor of Connecticut, but he never pursued the office.
Blumenthal announced his 2010 run for U.S. Senate after Democratic incumbent Chris Dodd announced his retirement. He faced professional wrestling magnate Linda McMahon in the general election, winning by a 12-point margin with 55 percent of the vote. In January 2011 he was sworn in and took seats on the Senate Armed Services; Judiciary; Aging; and Health, Education, Labor and Pensions Committees.