As a young man, Nelson had served in the United States military as a reservist, with two years in active duty. He was also an attorney and a member of the state legislature. In 1979, he was elected to the United States House of Representatives, where he served until 1991, being reelected in 1980, 1982, 1984, 1986 and 1988. In 1990 he was defeated in the Democratic primary for governor of Florida by Lawton Chiles, who later won the election. Nelson did not run for reelection for his house seat, but served in various positions in the state government for several years. In his 2000 Senate race, he took advantage of the retirement of Republican incumbent Cornelius McGuillicuddy, better known as Connie Mack and defeated his new Republican opponent.