William Thomas Pettit was born April 23, 1931, in Cincinnati, Ohio. He grew up in Waterloo, Iowa, and graduated from Waterloo West High School in 1949. He graduated with a bachelor's degree from the Iowa State Teachers College (now the University of Northern Iowa) in 1953, where he wrote for the student newspaper, the College Eye. He received a master's degree in American Studies from the University of Minnesota in 1958. He also received an honorary Doctor of Humane Letters from the University of Northern Iowa in 1988.

Pettit began his professional broadcasting career with WOI-TV in Ames, Iowa, in 1953. After two years he moved to KCRG-TV in Cedar Rapids, Iowa, and then to WCCO-TV in Minneapolis. He joined NBC in 1959 with WRCV-TV in Philadelphia, and, in 1962, moved to the NBC Los Angeles bureau. In 1968, he became chief West Coast correspondent for the National Education Television's Public Broadcast Laboratory. Later that year, he rejoined NBC as a Los Angeles correspondent. He served as an NBC correspondent at the Washington, D. C., bureau from 1975 through 1982. In March 1982, he became Executive Vice President of NBC News in New York. He served in that position until October 1985. He then served as Chief National Affairs correspondent from 1985 until 1989. From 1989 through the summer of 1992, he was an NBC correspondent based in London. He retired from NBC in 1995.

Pettit won three Emmy awards, a Peabody Award, the American Medical Association's Medical Journalism Award, and the Polk Memorial Award during his professional career. He interviewed all United States Presidents from Harry Truman through Bill Clinton. He was well-known for his on-the-scene reporting of Jack Ruby's murder of Lee Harvey Oswald in Dallas, Texas, in 1963. Pettit and his first wife, Betty T. Pettit, from whom he was divorced in 1989, had four children: Debra, Anne, James, and Robert. Tom Pettit later married Patricia Barry Pettit. Tom Pettit died in December 22, 1995.