Trimble attended St. Albans Prep School in Washington, D.C., and he was known for being a great athlete at any sport that he played. He enjoyed playing football during the off season in which he was the captain of his football team. Trimble's love was for baseball and he had the ability to throw and captivate the stunned onlookers with his tremendous fastball and paralyzing hard breaking curveball. Trimble never was defeated on the mound at St. Albans and pitched three no-hitters in his short career. He was considered one of the finest prospects during his time. In his personal life, Trimble was named by the writer Gore Vidal as the love of his life; the two met as classmates at St. Albans and were boyhood friends. The writer later dedicated his second novel, The City and the Pillar to "JT"; Vidal confessed in his memoir Palimpset that the initials were indeed Trimble's.
Trimble's throwing ability caught the attention of Senators owner, Clark Griffith. Griffith invited Trimble to a tryout with the Washington Senators during his senior year in May 1943. Griffith was so impressed by Trimble's mighty throwing style that he wanted to sign Trimble on the spot. Due to the fact that Trimble was only 17 years old, his mother insisted that he finish school before he could play baseball. To accommodate Trimble's mother's wishes, Griffith signed him to a $5,000 bonus contract and agreed to pay for the four-year scholarship at Duke University that was one of the top baseball powerhouse in the country. Trimble would play under the great coach Jack Coombs, who was a retired major league pitcher.
While attending Duke University, at the age of 18 Trimble was disqualified from officer training school because of defective sight in one eye. He declined to use his political contacts in Washington to get a waiver, and instead opted to enter the Marine Corps as an enlisted man. Private Trimble attended basic training at Parris Island, South Carolina. While attending Camp Lejeune, Private Trimble caught the attention of his superiors with his pitching ability. Pvt. Trimble was offered to pitch for the Camp Lejeune baseball team for four months and pick up the rank of corporal. If Pvt. Trimble had accepted this offer it would have kept him from being deployed into the battle field for the duration of his military career. Pvt. Trimble refused, stating that he joined the Marine Corps to fight the war against the enemy.
In July 1944, Trimble was deployed to join the Third Marine Division in the South Pacific. Trimble first saw combat on Guam as part of patrol missions for removing any remains of resistance Japanese soldiers. Once hostility ceased on the island, the military used sport to maintain the morale of the men in uniform. Baseball leagues and exhibition games were organized whenever conditions permitted, and Guam became a baseball hub in the Pacific. He pitched in the "Little World Series" held on Guam in 1944, and achieved considerable notoriety in military circles for his pitching skills, and his record for them was 6-2. Trimble became the star pitcher for the 3rd Marine Division Baseball team that was stationed at Camp Witek on Guam. Many of the professional baseball players were stationed on the Pacific, Trimble had great success pitching against them including a string of 21 straight wins in the Marine Corps.
On February 19, 1944, Pvt Trimble was a part of the division's Amphibious Reconnaissance Company, assigned directly to division headquarters. The Third Marine Division took heavy casualties from Japanese rocket attacks launched from the hill that came to be known as Hill No. 362. Pvt Trimble volunteered to serve in an elite eight man scouting platoon that would put him ahead of the front lines on Iwo Jima under the command of Major General Graves Erskine. Pvt Trimble's mission was to find the location of the rocket sites and to call in artillery to destroy them. The following night, four two-man reconnaissance team were in foxholes ahead of the rest of the platoon as an over watch. At midnight a flare signaled an attack, and immediately Japanese soldiers breached the marine's foxhole front line and were over running the marines in the fighting holes. During the battle Pvt Trimble was bayoneted on his right shoulder blade and continuing on with the fight when he was hit by a grenade that severely wounded him. Moments later a Japanese soldier with an explosive charge strapped to his waist, jumped in the foxhole and wrapped himself around Pvt Trimble and detonated the mine, killing himself and the young marine.
When told of Trimble's death, General Erskine was reported to have been "moist-eyed." Two months later, at a ceremony on Guam, the Third Division baseball field was named in memory of Jimmy Trimble by the personal order of Erskine, who was also wounded on Iwo Jima. The general himself attended the ceremony, a highly unusual honor rendered to a deceased private, and wrote the citation that was read to those attending. It said in part: "Private Trimble was an outstanding member of the Third Marine Division All-Star baseball team. Private Trimble's unswerving courage, loyalty, devotion to duty, and high ideals on and off the battlefield will long be remembered by his colleagues."