[3] He was commissioned a second lieutenant in the United States Army through the Reserve Officers Training Corps program at the University of California, Berkeley, where he graduated in May 1938.

A dissenter from General William Westmoreland's more conventional war strategy, Weyand's experience as a former intelligence officer gave him a sense of the enemy's intentions.

Weyand managed to convince a reluctant General Westmoreland to allow him to redeploy troops away from the Cambodian border area closer to Saigon, significantly contributing to making the 1968 Tet Offensive a military catastrophe for North Vietnam.

[4] Before the 1968 holiday truce for Tet went into effect, Fred Weyand got the feeling that "something was coming that was going to be pretty bad, and it wasn't going to be up on the Laotian border somewhere, it was going to be right in our own backyard.

"[5] Westmoreland's obsession with the enemy hitting the Marines at Khe Sanh turned out to be a tactical feint, and the Communist strategy all along was a multi-pronged, simultaneous attack of key cities (Hue', Da Nang, Nha Trang, Quinhon, Kontum, Banmethuot, My Tho, Can Tho, Ben Tre and Saigon) [6] Weyand's disposition of his forces denied the Communists from taking Saigon.

Weyand succeeded General Creighton Abrams, who was appointed as Army Chief of Staff, as commander of MACV on 30 June 1972.

By the end of 1972 General Weyand had overseen the withdrawal of all United States military forces from South Vietnam.

In an editorial in The New York Times on 11 December 2006, Murray Fromson, a reporter for CBS during the Vietnam War, stated that General Weyand had agreed to reveal himself as the confidential source for New York Times reporter R. W. Apple Jr.'s 7 August 1967, story "Vietnam: The Signs of Stalemate.

"[8] General Weyand, then commander of III Corps in Vietnam, was the unidentified high-ranking officer who told Apple and Fromson (reporting the same story for CBS) that: I've destroyed a single division three times ...

[9] [10] His awards and decorations include:[11] After retiring from the United States Army in 1976, Weyand moved to Honolulu, Hawaii, which was also the home of the 25th Infantry Division.

He became active in Hawai'i community affairs and held a number of prominent business positions, including corporate secretary and senior vice president of First Hawaiian Bank between 1976 and 1982.