Sandy Bonnyman was born on May 2, 1910, in Atlanta, Georgia. He attended Princeton University from 1928 to 1930, and enlisted in the Aviation Cadet Program of the U.S. Army Air Corps on June 28, 1932. After attending Preflight School at Randolph Field, Texas, he received an honorable discharge from the Army Air Corps on September 19, 1932. He next worked in the coal mining and copper mining industries before enlisting in the U.S. Marine Corps in July 1942. Pvt Bonnyman completed basic training at the Marine Corps Recruit Depot, San Diego, California, in October 1942, and then deployed to New Zealand aboard the SS Matsonia with the 6th Marine Regiment of the 2d Marine Division. He deployed with his unit to Guadalcanal in January 1943, and he was field promoted to 2dLt in February 1943. Lt Bonnyman then returned to New Zealand with his unit, and there he was promoted to 1stLt on September 1, 1943, and joined the 2d Battalion of the 8th Marine Regiment. He landed on Tarawa with his unit on D-Day, November 20, 1943, and he was killed in action 2 days later, on November 22, 1943. Lt Bonnyman was posthumously awarded the Medal of Honor for his actions on Tarawa, and the medal was presented to his daughter by Secretary of the Navy James F. Forrestal in Washington, D.C., on January 22, 1947. Lt Bonnyman was originally buried in Cemetery 27 on Tarawa, but the location of his grave was lost soon after the battle. His remains were found in June 2015, were repatriated to the United States on July 26, 2015, and were positively identified on August 27, 2015. He was buried at the Berry Highland Memorial Cemetery in Knoxville, Tennessee, on September 27, 2015.

His Medal of Honor Citation reads:

For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty as Executive Officer of the Second Battalion Shore Party, Eight Marines, Second Marine Division, during the assault against enemy Japanese-held Tarawa in the Gilbert Islands, from 20 to 22 November 1943. Acting on his own initiative when assault troops were pinned down a the far end of Betio Pier by the overwhelming fire of Japanese shore batteries, First Lieutenant Bonnyman repeatedly defied the blasting fury of the enemy bombardment to organize and lead the besieged men over the long, open pier to the beach and then, voluntarily obtaining flame throwers and demolitions, organized his pioneer shore party into assault demolitions and directed the blowing of several hostile installations before the close of D-Day. Determined to effect an opening in the enemy's strongly organized defense line the following day, he voluntarily crawled approximately forty yards forward of our lines and placed demolitions in the entrance of a large Japanese emplacement as the initial move in his planned attack against the heavily garrisoned, bombproof installation which was stubbornly resisting despite the destruction early in the action of a large number of Japanese who had been inflicting heavy casualties on our forces and holding up our advance. Withdrawing only to replenish his ammunition, he led his men in a renewed assault, fearlessly exposing himself to the merciless slash of hostile fire as hie stormed the formidable bastion, directed the placement of demolition charges in both entrances and seized the top of the bombproof position, flushing more than one hundred of the enemy who were instantly cut down and effecting the annihilation of approximately one hundred and fifty troops inside the emplacement. Assaulted by additional Japanese after he had gained his objective, he made a heroic stand on the edge of the structure, defending his strategic position with indomitable determination in the face of the desperate charge and killing three of the enemy before he fell, mortally wounded. By his dauntless fighting spirit, unrelenting aggressiveness and forceful leadership throughout three days of unremitting, violent battle, First Lieutenant Bonnyman had inspired his men to heroic effort, enabling them to beat off the counterattack and break the back of hostile resistance in the sector for an immediate gain of four hundred yards with no further casualties to our forces in this zone. He gallantly gave his life for his country.