In early 1945, while Robinson was at Sam Huston College, the Kansas City Monarchs sent him a written offer to play professional baseball in the Negro leagues. Robinson accepted a contract for $400 ($ in 2021 dollars) per month. This was a big deal for him at the time. He played well for the Monarchs, but Robinson was upset with the experience. He had grown used to having a structure while playing in college. The Negro leagues' lack of organization and acceptance of gambling interests bothered him. The travel schedule also placed stress on his relationship with Isum. The two could now communicate only by letter. In all, Robinson played 47 games at shortstop for the Monarchs. He hit .387 with five home runs and had 13 stolen bases. He also played in the 1945 Negro League All-Star Game (where he had no hits in five at-bats).
During the season, Robinson tried for a possible major league interest. The Boston Red Sox held a tryout at Fenway Park for Robinson and other black players on April 16, 1945. The tryout, however, was an act held mostly to make powerful Boston City Councilman Isadore Muchnick happy. Even with the stands limited to management, Robinson was subjected to racial comments. Robinson left the tryout humiliated. More than fourteen years later, in July 1959, the Red Sox became the last major league team to integrate its roster.
Other teams, however, had more serious interest in signing a black ballplayer. In the mid-1940s, Branch Rickey, club president and general manager of the Brooklyn Dodgers, began to scout the Negro leagues for a possible addition to the Dodgers' roster. Rickey selected Robinson from a list of African-American players. He interviewed Robinson for possible assignment to Brooklyn's International League farm club, the Montreal Royals. Rickey was especially interested in making sure his eventual hire could put up with the racial abuse that he would receive. In a famous three-hour discussion on August 28, 1945, Rickey asked Robinson if he could face the racial hatred without reacting angrily. This was a worry because of Robinson's past arguments with law enforcement officials at PJC and in the military. Robinson was shocked: "Are you looking for a Negro who is afraid to fight back?" Rickey replied that he needed a Negro player "with guts enough not to fight back." After receiving a vow from Robinson to "turn the other cheek" to racial taunts, Rickey agreed to sign him to a contract for $600 a month.
He made Robinson keep the agreement a secret for the time being. Rickey committed to officially signing Robinson before November 1, 1945. On October 23, it was announced that Robinson would be assigned to the Royals for the 1946 season. On the same day, with officials of the Royals and Dodgers present, Robinson signed his contract with the Royals. In what was later referred to as "The Noble Experiment", Robinson was the first black baseball player in the International League since the 1880s. Robinson was not necessarily the best player in the Negro leagues. Black players Satchel Paige and Josh Gibson were upset when Robinson was selected first.
Rickey's offer allowed Robinson to leave the Monarchs and their long bus rides behind. He went home to Pasadena.
That September, he signed with Chet Brewer's Kansas City Royals. This was a post-season barnstorming team in the California Winter League. Later that off-season, he toured South America with another team. His fiancée Isum worked as a nurse in New York City while he was away. On February 10, 1946, Robinson and Isum were married by their old friend, Rev. Karl Downs.
In 1946, Robinson arrived at Daytona Beach, Florida, for spring training with the Montreal Royals of the Class AAA International League. Robinson's being there upset people in racially sensitive Florida. He was not allowed to stay with his teammates at the team hotel. Instead he lived at the home of a local black politician. Since the Dodgers team did not own a spring training complex, the schedule was controlled by the towns in the area. Some of these towns did not allow any event involving Robinson or Johnny Wright, another black player whom Rickey had signed to the Dodgers in January. In Sanford, Florida, the police chief said he would cancel games if Robinson and Wright did not stop training there. Because of this, Robinson was sent back to Daytona Beach. In Jacksonville, the stadium was locked without warning on game day. This was ordered by the city's Parks and Public Property director. In DeLand, a day game was canceled, supposedly because of bad electrical lighting.
After a lot of talking to local officials by Rickey, the Royals were allowed to host a game involving Robinson in Daytona Beach. Robinson made his Royals debut at Daytona Beach's City Island Ballpark on March 17, 1946. It was an exhibition game against the Dodgers. With the game Robinson became the first African-American to openly play for a minor league team and against a major league team since the baseball color line had been put in place in the 1880s. Later in spring training, after some somewhat poor performances, Robinson was moved from shortstop to second base. This allowed him to make shorter throws to first base. Robinson's performance soon improved. On April 18, 1946, Roosevelt Stadium hosted the Jersey City Giants' season opener against the Montreal Royals. This game was the first professional game for the Royals' Jackie Robinson. In his five trips to the plate, Robinson had four hits, including a three-run home run. He also scored four runs, drove in three, and stole two bases in the Royals' 14–1 win. Robinson went on to lead the International League that season with a .349 batting average and .985 fielding percentage. He was named the league's Most Valuable Player. Although he often faced hatred while on road trips (the Royals were forced to cancel a Southern tour, for example), the Montreal fans supported Robinson. Whether fans supported or opposed it, Robinson's being on the field helped attendance. More than one million people went to games that Robinson played in 1946. The number was an amazing amount for the International League. In the fall of 1946, following the baseball season, Robinson returned home to California and briefly played professional basketball for the Los Angeles Red Devils.
The following year, six days before the start of the 1947 season, the Dodgers brought Robinson up to the major leagues. Eddie Stanky was playing second base for the Dodgers. So Robinson played his first major league season as a first baseman. On April 15, 1947, Robinson played his first major league game at Ebbets Field in front of a crowd of 26,623 spectators. More than 14,000 black fans attended the game. He did not get a base hit, but the Dodgers won 5–3. Robinson became the first player since the 1880s to openly break the major league baseball color line. Black fans began coming to see the Dodgers when they came to town, ignoring their Negro league teams.
Robinson's rise to the major leagues met a generally positive, although mixed, reception from newspapers and white major league players. However, there was racial tension in the Dodger clubhouse. Some Dodger players implied they would sit out rather than play alongside Robinson. The possible problem ended when Dodgers bosses defended Robinson. Manager Leo Durocher told the team, "I do not care if the guy is yellow or black, or if he has stripes like a fuckin' zebra. I'm the manager of this team, and I say he plays. What's more, I say he can make us all rich. And if any of you cannot use the money, I will see that you are all traded."