Prior to his election to the State Senate, he was mayor of West Hartford, Connecticut, serving on the Town Council for five years.

Elected to the State Senate in 1986,[2] Sullivan chaired the General Assembly's Education Committee.

Sullivan, as the Senate President Pro Tempore, succeeded to the office of Lieutenant Governor in 2004 where he served until 2007.

During his time as Lieutenant governor, he led a statewide effort to improve mental health care.

In 2011, Governor Dannel Malloy appointed Sullivan as Connecticut's Commissioner of Revenue Service.