After the war, his brothers formed the James-Younger gang with Frank and Jesse James. For ten years the gang robbed banks, trains, and stage coaches across Missouri, Kansas, and other nearby states. Bob Younger is believed to have first joined the gang in 1873.
In September of 1876 the gang robbed a bank in Minnesota and killed a clerk. The townspeople decided to fight back, and in the ensuing shootout all three of the Younger brothers were captured. Bob was wounded in the elbow and later in the chest.
Bob Younger was convicted and sentenced to life imprisonment. He died of tuberculosis in prison on September 16th, 1889 aged 35 years. His body lies buried in the Lee's Summit Historical Cemetery in Lee's Summit, Missouri.