Byrnes was born in Green Bay, Wisconsin, the son of Harriet (Schumacher) and Charles W. Byrnes.^[2] Byrnes contracted polio as a small child.^[3] He received his bachelor's degree from the University of Wisconsin in Madison in 1936, and then attended its law school.^[4] After graduation, Byrnes practiced law in Green Bay, and served as deputy commissioner of the state's department of banking from 1938 to 1941. He served in the state senate from 1941 to 1945, representing the 2nd District.^[5]

Byrnes was elected in 1944 as a Republican to the 79th Congress from Wisconsin's 8th district. He defeated one-term incumbent LaVern Dilweg, a notable former professional football player with the Green Bay Packers. Byrnes was reelected to the thirteen succeeding Congresses, and served 28 years from January 1945 to January 1973, making him the 8th district's longest serving representative. In 1964, the Wisconsin delegates to the 1964 Republican National Convention were pledged to support Byrnes, but he released the delegates to support the candidates they individually chose.^[6] He did not seek a fifteenth term in 1972 to the 93rd Congress. Byrnes voted in favor of the Civil Rights Acts of 1957,^[7] 1960,^[8][9] 1964,^[10][11] and 1968,^[12][13] as well as the establishment of the Wisconsin Islands Wilderness,^[14] the 24th Amendment to the U.S. Constitution, and the Voting Rights Act of 1965.^[15][16][17]

After Congress, stayed in Washington to practice law and continued to reside in Arlington, Virginia. While traveling to Wisconsin for a medical check-up at the Marshfield Clinic, he had a stroke at the Central Wisconsin Airport in Mosinee^[18] and died several days later in Marshfield on January 12, 1985.^[3][19] He was buried in Green Bay at Allouez Catholic Cemetery.

• United States Congress. "John W. Byrnes (id: B001216)". Biographical Directory of the United States Congress.
• Byrnes at Politicalgraveyard.com
• John W. Byrnes at Find a Grave