Between 2002 and 2006, she appeared as Jessica Baker in Cheaper by the Dozen and its sequel, as well as Lucy Miller in the second and third films of The Santa Clause trilogy.
In animation, Mumy performed the voice of Mertle Edmonds in the Lilo & Stitch franchise as well as Twinkle in Higglytown Heroes.
Liliana appeared with her father, Bill Mumy, in the Twilight Zone revival series' episode "It's Still a Good Life".
Lesser roles included TV appearances playing Rachael/Rachel in six episodes on My Wife and Kids from 2002 to 2004 as well as young Donna on That '70s Show and guest starring in Scrubs as a girl at her birthday party (whose face J.D.
In June 2012, Mumy started voice work as 'Beth' in Bravest Warriors, an animated series created by Pendleton Ward, produced by Frederator Studios for their channel, Cartoon Hangover, a premium content partner of YouTube.