Valentine played in Lincoln bands Kid Quarkstar, Mondello,and Happy Dog. In 2000, Happy Dog changed its name to Square and he moved to Los Angeles, California from Anaheim, California where he taught private guitar lessons out of his home.
Eventually the members of Square and Kara's Flowers became friends and when Kara's Flowers began looking for another guitarist, Valentine was approached. Personal tensions in Square helped the decision, and shortly afterward, Kara's Flowers became Maroon 5.
Valentine is a friend of John Mayer, whom he met in 1996 at a week long guitar school at Berklee in Boston, Massachusetts. Valentine once tried to get Mayer to move to Lincoln to play guitar for the Lincoln blues rock band Baby Jason and the Spankers. An opening slot on Mayer's 2004 summer tour helped bring widespread exposure to Maroon 5. Valentine contributed guitar work to Mayer's September 2006 release Continuum on the songs "Stop This Train" and "In Repair."
Valentine was raised as a member of The Church of Jesus Christ of Latter-day Saints (Mormon) and has three sisters and one brother. His family now lives in Utah, and both parents work for Brigham Young University, which is owned and operated by the LDS church. Valentine does not consider himself to be an active Mormon. His mother is a former beauty queen.
Before finding fame with Maroon 5, Valentine had also played with Reel Big Fish, firstly when guitarist Aaron Barrett broke his hand, and then later in 2001 when Barrett fell down some stairs. Valentine displayed a picture from one Reel Big Fish show he played during Maroon 5's appearance on MTV Cribs.