Hartnett was born in Charleston. He graduated from Bishop England High School in Charleston in 1960. He attended the College of Charleston from 1960 to 1961 and was in the United States Air Force Reserve from 1963 to 1969 and South Carolina Air National Guard from 1981 to 1987.

In 1964, Hartnett was elected to the South Carolina House of Representatives from a Charleston-area district. He served four terms in that body. Originally a Democrat, he became a Republican in 1972, and attended that year's state Republican convention (and every convention after that until 1980). He was elected to the South Carolina Senate in 1972 and served two terms.

In 1980, Hartnett won the Republican nomination for the Charleston-based 1st District after five-term incumbent Mendel Jackson Davis retired due to back problems. He narrowly defeated his Democratic opponent, Associate Deputy Commerce Secretary Charles D. Ravenel, becoming the first Republican to win an undisputed election in the district since Reconstruction. Hartnett likely owed his win to Ronald Reagan winning Charleston County with 55% of the vote. The district had also been trending Republican for some time at the national level; it has only supported the Democratic candidate for president once since 1956, when Jimmy Carter carried it in 1976. But conservative Democrats continued to hold most of the district's seats in the state legislature, as well as most local offices, well into the 1990s.

Hartnett was convincingly reelected in 1982, and took 61% of the vote in 1984. He gave up his seat in 1986 to run for lieutenant governor, narrowly losing to Democratic State Senator Nick Theodore. He then became a real estate agent, founding Hartnett Realty in his hometown of Mount Pleasant.

Hartnett came out of retirement in 1992 to run for the United States Senate against four-term incumbent and fellow Charleston resident Ernest Hollings. He gave Hollings his closest race ever, losing by only three percentage points in a very good year for Democrats nationally.

Hartnett lives in Mount Pleasant and is chairman of the family-owned Hartnett Realty.^[1] The firm was created in 1947 by Catherine Forbes Hartnett and is one of the oldest^[2] Charleston real estate firms. He was a delegate to every Republican National Convention from 1980 to 2000. His son, Tom Hartnett Jr., currently serves in the South Carolina House of Representatives.

• United States Congress. "Thomas F. Hartnett (id: H000302)". Biographical Directory of the United States Congress.
• Appearances on C-SPAN