[7] In 1976 he ran for the Massachusetts State Senate, losing the Democratic primary to incumbent William X.

[8][9] He was elected to the U.S. House of Representatives in 1978, succeeding Paul E. Tsongas, who ran successfully for the United States Senate.

He was defeated in the Democratic primary by Lieutenant Governor John Forbes Kerry, who went on to win the seat.

Shannon served out the rest of his term in the House, leaving office in January 1985, at which point he was elected to the Common Cause National Governing Board.

[14][15] He was elected president and Chief Executive Officer of the NFPA (National Fire Protection Association) in 2002, and served until 2014.