Kelly made her professional acting debut in the fact-based made-for-TV movie
Love, Lies and Murder, playing teenager Cinnamon Brown, who was coerced by her father into killing his wife and her stepmother, Linda Brown. She was originally going to have a starring role as
Polly Pry in
Trey Parker and
Matt Stone's
Cannibal! The Musical, but was convinced not to do so by her agent out of concerns it could ruin her career, and was ultimately replaced by
Toddy Walters, though she was still credited in the end credits under her initials "M.K." as "the Dropout". She went on to have small roles in the films
The Boy Who Cried Bitch,
Hi-Life, and
Billy Bathgate before being cast as
Donna Hayward in
Twin Peaks: Fire Walk with Me. For that film, she went home and got permission from her priest because of an explicit sex scene. She has since appeared in the films
With Honors,
Little Odessa,
The Tie That Binds, and
Dangerous Beauty, amongst others, and provided the adult voice of
Nala in
Disney's
The Lion King, ''
The Lion King II: Simba's Pride, and The Lion King 1½. In her independent film career, Kelly had the starring role of activist Dorothy Day in Entertaining Angels: The Dorothy Day Story and starred alongside Glenn Close in The Safety of Objects. She played Helen Keller in the made-for-TV movie Monday After the Miracle'', which broadcast on November 15, 1998, on
CBS. Kelly starred in the CBS drama
To Have & to Hold opposite
Jason Beghe before playing
Mandy Hampton in the first season of
The West Wing. In 2003, Kelly began playing single mother
Karen Roe on the teen drama
One Tree Hill. She also directed two episodes of the series: "Resolve" (2007) and "I Slept with Someone in Fall Out Boy and All I Got Was This Stupid Song Written About Me" (2006). In the fifth season, she ceased to be a regular cast member, but made guest appearances in the 100th episode and the sixth-season finale. She has made guest appearances in television shows such as
Heroes,
Law & Order, and
Numb3rs. Kelly has also appeared in the films
Remember the Daze,
A Smile as Big as the Moon,
Taken Back: Finding Haley, and
Girl in the Bunker. Kelly reprised her role of
Nala from the Disney animated movie series
The Lion King in adventure video game
Disney Dreamlight Valley released in 2023. ==Personal life==