Majorino was born in Westlake, California to Robert Majorino and Sarah Black; she has an older brother, Kevin, and is of part Italian ancestry.

Majorino began her acting career in 1992's television series, Camp Wilder, before being cast in her first film role in 1994's When a Man Loves a Woman. She subsequently starred in the leading roles of the family films Corrina, Corrina and Andre, both of which were released in August 1994. Her next film was 1995's action film, Waterworld, playing a child rescued by Kevin Costner's character. Most of her roles throughout the late 1990s were in independent films or made-for-television productions; she played Alice in the 1999 television film, Alice in Wonderland.

After Alice, she did not appear in another film until 2004, when she starred opposite Jon Heder in the movie Napoleon Dynamite. Between 2005 and 2006, she played a recurring role on UPN's series, Veronica Mars, as the computer-savvy Mac. She also appeared in Blind, a music video by the rock group, Lifehouse.