Appleby began her acting career at the age of four, starting with advertising for various products such as Cheerios and M&M's. Her first advertisement was for Raisin Bran but it was never aired. She has had guest appearances on many television programs, most notably thirtysomething (1987), Doogie Howser, M.D. (1989), ER (1994), Baywatch (1989), Xena: Warrior Princess (1995), 7th Heaven (1996) and Beverly Hills 90210 (1990). For Roswell, she tried out for the roles of both Isabel and Maria before landing the role of Liz Parker. On 7th Heaven, she played a gang member who hid weapons underneath her little brother's mattress.

Appleby was in Bon Jovi's music video for "It's My Life" with Will Estes. She was also featured in the 2004 music video for the song "I Don't Want To Be" by Gavin DeGraw, opposite Scott Mechlowicz. She also appeared in Sense Field's music video for "Save Yourself" which is part of the Roswell soundtrack and featured in the first season DVD boxset.