Derwinski served in the United States Army in the Pacific Theater during World War II and in the postwar U.S. occupation of Japan. He graduated from Loyola University Chicago in 1951. He was a member of the Alpha Delta Gamma Fraternity. In 1957 he was elected to the Illinois House of Representatives, where he served one term before winning election to the U.S. House of Representatives in 1958. He served 12 terms as a Republican representative from the 4th District of Illinois, a suburban region south and west of Chicago, eventually becoming the ranking member of the House Foreign Relations Committee. He was also a delegate to the United Nations General Assembly from 1971-1972 and as chairman of the U.S. delegation to the Interparliamentary Union from 1970-1972 and 1978-1980.