"A horrible day, I was there that day," wrote Rick Lose, describing Phinn's death.

William Mark Phinn was born to Mr. and Mrs. William A. Phinn on June 25, 1948. Information regarding his early life, education or arrival in Santa Cruz County is not known.

In 1968 Phinn's family, which also consisted of his brother Michael, was living near the Capitola Mall in the mid-county area.

William Phinn enlisted in the US Marine Corps in May 1968, and was sent to Camp Pendleton, California for his basic training. While there a fellow recruit, Herb Grimaud, recalled that:
“Bill, me and 70 odd other civilians were brought together at MCRD [Marine Corps Recruit Depot] San Diego, April 1968 thru June 1968. It was Platoon 279 and we were the Series Honor Platoon. That means that out of three other platoons in our company that went through boot camp at the same time, we won almost all of the final competitions it took to graduate. Things like drill, rifle range, physical fitness, academic, first aid, etc. We "smoked" everybody. We were pretty raggedy when we first started but we were really squared away by the last few weeks of boot camp.”

Following basic training, Private Phinn completed school-of-infantry training at the San Diego facility and after graduating, returned home for a short leave before departing for Vietnam.

William Phinn arrived in Vietnam on October 5, 1968, and likely was assigned to the 3rd Battalion, 26th Regiment of the 3rd Marine Division in Quang Nam province. * He participated in a number of sweeps during Operation Dewey Canyon prior to his final patrol.

Squad Leader, Rick Lose, recalled the events of March 18, 1969, the day of Phinn's death on the Vietnam Wall Internet Website.
“A horrible day, I was there that day. We were on a search and destroy mission all day [and] were hit hard [but] we did not take any losses. When we returned that afternoon LCpl Phinn was removing his frags [fragmentation grenades] from his ammo bag and it happen[ed] [that] a frag lost its pin and exploded killing four Marines. Phinn did not suffer at all he went peacefully to Jesus. He was a good marine and loved his country.”

Lance Corporal William Mark Phinn's body was recovered and returned to California for burial in the Golden Gate National Cemetery in San Bruno.

(VVMW; USDVA; SCSn March 21, 1969 1:1; Operation Dewey Canyon, http://recruitknowledge.com/pages/history/mch5.htm [16 September 2009])

* Note and possible correction:
Karl Franklin Wilkinson states "[Robert L. Nelson, author] notes that William Phinn was "likely assigned to 3rd Battalion, 26th Regiment, 3rd Marine Division... "that isn't accurate. He was in my platoon, 1st Platoon, 2nd Battalion, 5th Marine Regiment... the most decorated Battalion in the Marine Corps! He was my closest friend during those hard days and always helped bring smiles and laughter to all of us."
[Email to SCPL from Karl Franklin Wilkinson, 12 October, 2021]