Allen was born in Ventura, California to Dalton Earl Allen, Sr. and Katherine Mary Raposa, who later divorced. Allen's paternal grandparents, Roy Areatus Allen and Ima Jean Flenniken, were several-generation Texans, while her maternal grandfather, Antone Pacheco Raposa, was of Portuguese origin and her maternal grandmother, Mary Elizabeth Nolan, was of English and Irish ancestry. Allen has an older brother, Dalton Earl Allen, Jr., and lived in Austin, Texas, where she went to school at Austin Community College and majored in education. She then interned as a kindergarten teacher before she decided to pursue her dream of an acting career.

Allen appeared on the television shows Days of Our Lives (as Billie Reed, 1996-1999) and Baywatch (Jenna Avid, 2000-2001). Allen has also appeared in films like Confessions of a Dangerous Mind, Anger Management, and the Project Greenlight horror film Feast. Allen is also well known for playing a sexy virtual reality character in an episode of The X-Files and is remembered for her brief appearance as the "Elevator Girl" in the Jim Carrey comedy Liar Liar.

In both Smallville and Mutant X, Allen played a woman who could control men with her pheromones. She also appeared in Toby Keith's video "A Little Too Late" and had a small role in CSI as a hooker who caught the eye of Nick Stokes in the pilot episode: she returned later in the first season with tragic consequences.

Allen is featured in Maxim magazine's Girls of Maxim gallery and was Named #70 on the Maxim "Hot 100 of 2005" list. She has her own T-Shirt line called Sex Brand.

Krista will be doing stand-in radio presenter work on Invicta FM in Kent, Southern FM in Sussex and 103.2 Power FM in Hampshire.

Allen is sometimes credited as Krista Allen-Moritt as a result of her marriage to Justin Moritt which lasted from 1996 to 1999; they have 1 son: Jacob Nolan, born on 12 July, 1997. She has also been romantically linked to actor George Clooney.