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After graduating from Sam Houston State University he worked in Austin as a real estate appraiser. In 1987, he quit his job and moved to Nashville to pursue a career as a song writer. He supported himself by parking cars and working in a warehouse. During live shows he tells stories about his parents sending him money so he could survive. In 1990, Randy Travis took a song Shamblin wrote, about his great-grandfather, to number one on the country charts. After "He Walked on Water", he followed it up with four more number one songs. He often co-writes with other songwriters. He co-wrote with Steve Seskin for number one hits with Life is a Dance and Don't Laugh at Me. "Don't Laugh at Me" was a hit for Mark Wills and was later recorded by Peter, Paul and Mary resulting in a school program designed to teach children tolerance and prevent bullying in the playground. His biggest song, "I Can't Make You Love Me," was co-written with Mike Reid and was a hit for Bonnie Raitt.
In 2009, Shamblin was inducted into The Texas Heritage Songwriters Association on March 1, 2009 at the Paramount Theater in Austin, Texas.