Despite the fact that he had no previous criminal record and his family was well respected in the community, he was given a prison sentence of 10 to 20 years. Singleton was eventually arrested also but, with the help of a lawyer, was released from prison after only two years. .
During Dillinger's nine years in prison, he became a hardened criminal. At the reformatory in Pendleton, Indiana, many of his cellmates were among the most notorious bank robbers in Indiana. Dillinger learned many of his skills as a bank robber from his cellmates and some of them would later become members of his gang. He was considered a model prisoner, although he did make one clumsy attempt to escape and was reprimanded several times for disorderly conduct. With five dollars in his pocket, he was released from the reformatory on May 22, 1933 and briefly returned to Mooresville. However, his wife had divorced him four years earlier and he saw no reason to stay in his hometown. On the night of July 16, 1933, at the age of 30, John Herbert Dillinger left Mooresville to become one of America's most famous criminals.
Beginning in mid-1933, Dillinger and his gang embarked upon a bank robbing spree. During the spree, the gang robbed 11 banks throughout the Midwest. Fifteen law enforcement officers and private citizens were killed during these robberies and 17 were wounded. Eleven members of Dillinger's gang were killed, including Harry Pierpont and Baby Face Nelson, and 23 were sent to prison. The gang netted roughly $300,000 from their robberies, a very impressive sum at the time. .
In January 1934, Dillinger and his gang were captured in Tucson, Arizona.Under heavy guard, Dillinger was transferred to Crown Point prison in Indiana. On March 3, 1934, Dillinger escaped. Using a wooden gun that had been smuggled into the prison, he overpowered the guards, locked them up, and walked through the prison gates. Dillinger's escape embarrassed law enforcement officials and made him a folk hero to some Americans.