Tom was born in 1940 in Marinette, Wisconsin, the son of  Navy Lieutenant Commander and Mrs. Thomas Evert. After his father was killed during a World War II mission over the Atlantic, Tom and his infant brother moved with his widowed mother, Marian, into a rented duplex in Fond du Lac, where she found a job teaching in the Fond du Lac public schools. In 1946, Marian married Robert Petri, who adopted the two boys. 

Tom attended Fond du Lac public schools from kindergarten through high school, graduating from Goodrich High in 1958. For two years, when Tom was about nine or 10, he earned trips to summer camp by selling several cases of Rosemond soap door to door. When he was older, he took jobs painting houses, delivering the old Fond du Lac Commonwealth-Reporter, working as a golf caddie and as a messenger and bank teller for the First Wisconsin Bank, and selling advertising and hosting the "Teen Time" show for KFIZ radio. 

Tom also attended Badger Boys State and Boys Nation. It was through those programs that he met President Eisenhower, Wisconsin Senator Alexander Wiley, and Rep. William K. Van Pelt, who represented the 6th District. He also met General Curtis LeMay. 

In 1961 Tom worked in the east African nation of Kenya for three months with Operation Crossroads, a church-supported humanitarian organization which served as one of the early models for President Kennedy's Peace Corps. 

In 1962 he graduated from Harvard College. In 1965 Tom earned a law degree from Harvard Law School. Then, in 1965-66, he clerked for James Doyle, the federal judge for Wisconsin's western district. 

In 1966-67, Tom served in Somalia, first with the Peace Corps and then with the United States Agency for International Development. In 1968 he was Executive Director of the Ripon Society. In 1969-70 Tom worked in the White House as Director of Crime Studies for President Nixon's Advisory Council on Executive Reorganization (also known as the "Ash Council"). His work focused on anti-drug efforts. 

Tom worked as a lawyer in Fond du Lac from 1970 to 1979, and was elected to the Wisconsin State Senate in 1973. Tom was the Republican nominee for the U.S. Senate in 1974 which, in the wake of Watergate, was a very bad year for Republicans. Although he lost, the Senate campaign gave Tom an opportunity to meet with all sorts of people and learn about their lives. And it gave him added visibility which helped when he ran in a special election in 1979 to replace Rep. Bill Steiger, who had died shortly after winning re-election in 1978. 

After winning the Republican primary, in which he gave the future Governor Tommy Thompson his one and only electoral defeat, he was elected to the U.S. House of Representatives after a hard fought campaign. He was re-elected in 1980 and has been re-elected every two years since.