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Mendenhall spent five successful years with the Pittsburgh Steelers before going to Arizona in 2013.
In an article on the Huffington Post, Mendenhall explained his decision.
He said he's retiring because the commercialization of football has forced him to become an entertainer rather than a football player.
He also said he's fed up with people saying racist things about him on the Internet.
He wrote:
"I've always been a professional. But I am not an entertainer. I never have been. Playing that I've always been a professional. But I am not an entertainer. I never have been. Playing that role was never easy for me. The box deemed for professional athletes is a very small box. My wings spread a lot further than the acceptable athletic stereotypes and conformity was never a strong point of mine. My focus has always been on becoming a better me, not a second-rate somebody else.
NFL running backs have notoriously short shelf lives. The average length of a running back's career is 3.11 years - the shortest of any NFL position.
Mendenhall tore his ACL in 2012, and hasn't been the same since. He averaged 3.2 yards per carry last year - his lowest average since his rookie year. He made $2.5 million in 2013, and would have been an unrestricted free agent.
Mendenhall says he's going to travel the world and write.