His career kicked off in 1959 at the City News Bureau of Chicago working as a copyboy and later as a crime reporter. He was then enlisted as an Army reservist. Seymour spent three months in basic training at Fort Leavenworth in Kansas in 1960. Seymour then worked as a correspondent for United Press International (UPI) in 1962 and moved back to AP in 1963. In 1965, he was transferred to its Washington, D.C., bureau to report on the Pentagon.
Moreoever, he reported on the intensifying U.S. involvement in the Vietnam War in 1966. After one year, he became part of the AP’s first special investigative unit. Seymour became a freelancer after his editors diminished a piece he wrote on the U.S.’s secretive chemical and biological weapons programs. He then authored six articles in national magazines in 1967.
For the first three months, he served as the press secretary for anti–Vietnam War candidate Senator Eugene McCarthy in his campaign in the 1968 Democratic Party presidential primaries.
