Sullivan was born in Omaha, Nebraska. His father, J. B. Sullivan, moved the family to Douglas, Wyoming, to open a law practice. Mike Sullivan graduated from Douglas High School as the class salutatorian. He continued his education at the University of Wyoming in Laramie, where he earned a bachelor's degree in petroleum engineering and then a law degree. On September 2, 1961, Sullivan married Jane Metzler of Riverton in ceremonies in Powell. They made their home in Casper. Sullivan practiced law with the firm of Brown, Drew, Apostolos, Massey, and Sullivan for twenty years. Sullivan had never sought elective office when, in 1986, he ran for and won the Democratic nomination for governor. Many observers believed that, following three terms with a Democratic governor the chances for another Democrat to gain that post would be remote. Nonetheless, after a hard-fought campaign, Sullivan defeated Republican nominee Peter K. Simpson. In his 1990 reelection, Sullivan defeated the rancher and businesswomanMary Mead of Jackson, daughter of Republican former Governor and U.S. Senator Clifford P. Hansen. Sullivan received 104,638 votes to her 55,471 ballots. Sullivan tapped the journalist Dennis E. Curran, a native of Wisconsin, to serve from 1987 to 1994 as his press secretary, after which time Curran launched the Wyoming Business Report. He named a Republican, Joseph B. Meyer, as attorney general. Sullivan ran for the U.S. Senate in 1994, but lost 59-39 percent to Representative Craig L. Thomas during the national Republican wave of 1994. Four years after his governorship ended, Sullivan was appointed as United States Ambassador to Ireland by President Bill Clinton, a post he held from 1999 to 2001. His official gubernatorial portrait was painted by artist Michele Rushworth and unveiled in the state capitol by Governor Dave Freudenthal in 2008. The portrait shows Sullivan holding his signature grey cowboy hat. In 2012, Sullivan was honored at the University of Wyoming with the dedication of a landscaped plaza bordering the university's Prexy's Pasture. Sullivan retired as a partner at the Casper office of the law firm of Lewis, Roca, Rothgerber, & Christie in 2015.
