Seaman was born in Manila, Philippines, the son of a U.S. Army officer about 1912. He was appointed to United States Military Academy, graduating with the class of xxxx.
He served in World War II in both European and Pacific theaters.
As a major general in 1965, Seaman was named commander of the 1st Infantry Division (United States) at Fort Riley, Kansas. The 1st Division was the first Army combat division to be called up for service during the Vietnam War, arriving in theater in July 1965, with Seaman being the first of six combat commanders during the war. In March 1966, Seaman was named commander of U.S. Field Force II, a 100,000 man fighting force that included three divisions and several independent brigades.
In 1967, he returned stateside and assumed command of First United States Army at Fort Meade, Maryland. In 1971, he retired to Beaufort, South Carolina after 37 years of active duty. Seaman died at the Veteran Administration Hospital in Charleston, South Carolina at age 74 on February 18, 1986.
He was married to Mary Grunert, sister of U.S. Army general and previous U.S. First Army commander, George Grunert.