| Edward Asner | Mr. Harris (voice) (as Ed Asner) |
| Angela Bassett | Miles' Mom (voice) |
| Lucas Black | Randy (voice) |
| Theodore Borders | Martin at age 12 (voice) |
| LeVar Burton | Martin at age 26 (voice) (as Levar Burton) |
| Jessica Marie Garcia | Maria (voice) (as Jessica Garcia) |
| Danny Glover | Train Conductor (voice) |
| Whoopi Goldberg | Mrs. Peck (voice) |
| Samuel L. Jackson | Turner (voice) |
| James Earl Jones | Daddy King (voice) |
| Ashley Judd | Mrs. Dale (voice) |
| Richard Kind | Mr. Willis (voice) |
| Dexter King | Martin at age 34 (voice) (as Dexter Scott King) |
| Yolanda King | Christine King (voice) |
| Zachary Leigh | Kyle (voice) |
| Robert Ri'chard | Miles (voice) |
| Susan Sarandon | Mrs. Clark (voice) |
| John Travolta | Kyle's Dad (voice) |
| Jaleel White | Martin at age 15 (voice) |
| Adam Wylie | Sam Dale (voice) |
| Oprah Winfrey | Coretta Scott King (voice) |
| Frank Welker | Bull Connor / Chihuahua (voice) |
| Jess Harnell | Reporter #1 / Demonstrator (voice) |
| Joe Lala | Reporter #2 / Demonstrator (voice) |
| John Wesley | Man / Demonstrator (voice) |
| Elizabeth Primm | Old Woman / Demonstrator (voice) |
| Jodi Carlisle | Additional (voice) |
| Ralph Abernathy | Himself - consoles Coretta King (archive footage) (uncredited) |
| Eugene 'Bull' Connor | Himself (archive footage) (uncredited) |
| Jesse Jackson | Himself - pallbearer (archive footage) (uncredited) |
| Ethel Kennedy | Herself - at funeral of King (archive footage) (uncredited) |
| Robert F. Kennedy | Himself - at funeral of King (archive footage) (uncredited) |
| Coretta Scott King | Herself (archive footage) (uncredited) |
| Martin Luther King | Himself (archive footage) (uncredited) |
| Bayard Rustin | Himself - on steps of Lincoln Memorial (archive footage) (uncredited) |
