He was an American criminal and outlaw, the younger brother of Cole and Jim Younger, he was a member of the James-Younger gang.

Born in Missouri on October 29, 1853, Robert was the thirteenth of fourteen children born to Henry Washington Younger and Bersheba Leighton Fristoe. During the Civil War his brothers Cole and Jim rode with Quantrill's Raiders. Bob was only 8 when the war broke out in 1861. He saw his father killed by Union soldiers and his home burned to the ground.

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.