Keith first worked with Neil Young in 1971 on Young's Harvest album, having been introduced by Elliot Mazer, the album's producer, who was looking for a session player in Nashville on short notice. This spawned a collaboration that would last nearly 40 years, as Keith went on to play with Young on over a dozen albums and numerous tours. Keith also played the role of Grandpa Green in the Neil Young feature length movie Greendale, a film accompaniment released on DVD to Young's 2004 album of the same name.
In addition to his work with Young, Keith also worked with Terry Reid, Todd Rundgren, Lonnie Mack, The Band, Blue, David Crosby, Graham Nash, Paul Butterfield, J. J. Cale, Linda Ronstadt, Warren Zevon, Ian and Sylvia, Emmylou Harris, Willie Nelson, Waylon Jennings, Anne Murray and Ringo Starr. He also served as the producer of Jewel's debut album Pieces of You, and worked as solo artist. He toured with Crosby Stills Nash & Young on their 2006 Freedom of Speech tour.
Keith died of a blood clot in his lung while at his home on Young's ranch in Northern California on July 26, 2010.