Jack Roosevelt Robinson was born on January 31, 1919, into a family of sharecroppers in Cairo, Georgia. He was the youngest of five children born to Mallie (née McGriff) and Jerry Robinson, after siblings Edgar, Frank, Matthew (nicknamed "Mack"), and Willa Mae. His middle name honored former President Theodore Roosevelt, who died 25 days before Robinson was born. After Robinson's father left the family in 1920, they moved to Pasadena, California.

The extended Robinson family established itself on a residential plot containing two small houses at 121 Pepper Street in Pasadena. Robinson's mother worked various odd jobs to support the family. Growing up in relative poverty in an otherwise affluent community, Robinson and his minority friends were excluded from many recreational opportunities. As a result, Robinson joined a neighborhood gang, but his friend Carl Anderson persuaded him to abandon it.
In 1935, Robinson graduated from Washington Junior High School and enrolled at John Muir Technical High School. Recognizing his athletic talents, Robinson's older brothers, Frank and Mack (himself an accomplished track and field athlete and silver medalist behind Jesse Owens in the 200 meters at the Berlin 1936 Summer Olympics) inspired Jackie to pursue his interest in sports.

At Muir Tech, Robinson played numerous sports at the varsity level and lettered in four of them: football, basketball, track and field, and baseball. He played shortstop and catcher on the baseball team, quarterback on the football team, and guard on the basketball team. With the track and field squad, he won awards in the broad jump. He was also a member of the tennis team.

In 1936, Robinson won the junior boys singles championship in the annual Pacific Coast Negro Tennis Tournament and earned a place on the Pomona annual baseball tournament all-star team, which included future Hall of Famers Ted Williams and Bob Lemon. In late January 1937, the Pasadena Star-News newspaper reported that Robinson "for two years has been the outstanding athlete at Muir, starring in football, basketball, track, baseball, and tennis."
After Muir, Robinson attended Pasadena Junior College (PJC), where he continued his athletic career by participating in basketball, football, baseball, and track. On the football team, he played quarterback and safety. He was a shortstop and leadoff hitter for the baseball team, and he broke an American junior college broad-jump record held by his brother Mack with a jump of 25 ft. 6+1⁄2 in. on May 7, 1938. As at Muir High School, most of Jackie's teammates were white. While playing football at PJC, Robinson suffered a fractured ankle, complications from which would eventually delay his deploym…

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In 1942, Robinson was drafted and assigned to a segregated Army cavalry unit at Fort Riley, Kansas. Having the requisite qualifications, Robinson and several other black soldiers applied for admission to an Officer Candidate School (OCS) then located at Fort Riley.

Although the Army's initial July 1941 guidelines for OCS had been drafted as race-neutral, few black applicants were admitted into OCS until after subsequent directives by Army leadership. The applications of Robinson and his colleagues were delayed for several months. After protests by heavyweight boxing champion Joe Louis (then stationed at Fort Riley) and with the help of Truman Gibson (then an assistant civilian aide to the Secretary of War), the men were accepted into OCS. The experience led to a personal friendship between Robinson and Louis. Upon finishing OCS, Robinson was commissioned as a second lieutenant in January 1943. Shortly afterward, Robinson and Isum were formally engaged.

After receiving his commission, Robinson was reassigned to Fort Hood, Texas, where he joined the 761st "Black Panthers" Tank Battalion. While at Fort Hood, Robinson often used his weekend leave to visit the Rev. Karl Downs, President of Sam Huston College (now Huston–Tillotson University) in nearby Austin, Texas; in California, Downs had been Robinson's pastor at Scott United Methodist Church while Robinson attended PJC.

An event on July 6, 1944, derailed Robinson's military career. While awaiting results of hospital tests on the ankle he had injured in junior college, Robinson boarded an Army bus with a fellow officer's wife; although the Army had commissioned its own unsegregated bus line, the bus driver ordered Robinson to move to the back of the bus. Robinson refused. The driver backed down, but after reaching the end of the line, summoned the military police, who took Robinson into custody. When Robinson later confronted the investigating duty officer about racist questioning by the officer and his assistant, the officer recommended Robinson be court-martialed.

After Robinson's commander in the 761st, Paul L. Bates, refused to authorize the legal action, Robinson was summarily transferred to the 758th Battalion—where the commander quickly consented to charge Robinson with multiple offenses, including, among other charges, public drunkenness, even though Robinson did not drink.

By the time of the court-martial in August 1944, the charges against Robinson had been reduced to two counts of insubordination during questioning. Robinson was acquitted by an all-white panel of nine officers.

Although his former unit, the 761st Tank Battalion, became the first black tank unit to see combat in World War II, Robinson's court-martial proceedings prohibited him from being deployed overseas, and he was never in combat.

After his acquittal, he was transferred to Camp Breckinridge, Kentucky, where he served as a coach for army athletics until receiving an honorable discharge in November 1944. While there, Robinson met a former player for the Kansas Cit…

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After his discharge, Robinson briefly returned to his old football club, the Los Angeles Bulldogs. Robinson then accepted an offer from his old friend and pastor Rev. Karl Downs to be the athletic director at Samuel Huston College in Austin, then of the Southwestern Athletic Conference. The job included coaching the school's basketball team for the 1944–45 season. As it was a fledgling program, few students tried out for the basketball team, and Robinson even resorted to inserting himself into the lineup for exhibition games. Although his teams were outmatched by opponents, Robinson was respected as a disciplinarian coach, and drew the admiration of, among others, Langston University basketball player Marques Haynes, a future member of the Harlem Globetrotters.