| Emile Hirsch | Chris McCandless |
| Marcia Gay Harden | Billie McCandless |
| William Hurt | Walt McCandless |
| Jena Malone | Carine McCandless / Additional Narrator |
| Brian H. Dierker | Rainey (as Brian Dierker) |
| Catherine Keener | Jan Burres |
| Vince Vaughn | Wayne Westerberg |
| Kristen Stewart | Tracy Tatro |
| Hal Holbrook | Ron Franz |
| Jim Gallien | Himself |
| James O'Neill | Graduation Reader (as James J. O'Neill) |
| Malinda McCollum | Waitress |
| Paul Knauls | Building Manager |
| Zach Galifianakis | Kevin |
| Craig Mutsch | Wayne's Crew #1 |
| Jim Beidler | Wayne's Crew #2 |
| John Decker | Hutterite #1 |
| John Hofer | Hutterite #2 |
| Jerry Hofer | Hutterite #3 |
| Terry Waldner | Hutterite #4 |
| Robin Mathews | Gail Borah |
| Candice Campos | Bar Girl |
| Steven Wiig | Lee's Ferry Ranger |
| Thure Lindhardt | Mads |
| Signe Egholm Olsen | Sonja |
| Floyd Wall | Man in Phone Booth |
| Bryce Walters | Chris McCandless - age 4 |
| Jim Davis | Immigration Officer |
| Cheryl Francis Harrington | Social Worker |
| R.D. Call | Bull |
| Haley Ramm | Carine McCandless - age 11 |
| Merritt Wever | Lori |
| Everett 'Insane Wayne' Smith | Insane Cain |
| John Jabaley | Announcer |
| Leonard Knight | Himself |
| Matt Contreras | Book Shopper #1 |
| Denise Sitton | Book Shopper #2 |
| Bart the Bear | Bear |
| Sharon Olds | Additional Narrator (voice) |
| Carine McCandless | Additional Narrator (voice) |
| Dan Burch | Passerby (uncredited) |
| Joe Dustin | The Beast (uncredited) |
| Golbon Eghtedari | Graduate (uncredited) |
| Alvin Ellie | Homeless Patron (uncredited) |
| Caroline Gelabert | Graduating Billie McCandless (uncredited) |
| Geri Gilmore | Old Lady Tourist (uncredited) |
| Paul Haasch | Trail Guide (uncredited) |
| Aaron Wayne Hill | Big Wheel Boy (uncredited) |
| Mad Martian | Guy At Bar (uncredited) |
| Parris Mosteller | Young Boy (voice) (uncredited) |
| Rocky Self | Punk Rocker (uncredited) |
| J. Nathan Simmons | Tourist (uncredited) |
| Susan Spencer | Parent (uncredited) |
| Haley Sweet | Bank Teller (uncredited) |
| Edward Tubbs | Brown Bronco Driver (uncredited) |
