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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 songwriter. 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 including: "We Were in Love," "In This Life" and "Walk on Faith." 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. His song "The House that Built Me", co-written with Tom Douglas, won a Grammy as sung by Miranda Lambert.
In 2009, Shamblin was inducted into The Texas Heritage Songwriters Association Hall of Fame on March 1, 2009 at the Paramount Theater in Austin, Texas. In 2011 he was inducted into the Nashville Songwriters Hall of Fame.