Disney Channel fans came to know her as the bubbly Lizzie McGuire on the series Lizzie McGuire from 2001 to 2004. She also starred in such popular films as The Lizzie McGuire Movie, A Cinderella Story and Cheaper by the Dozen.
She began acting at a young age, starring in a production of The Nutcracker at age 8.
She played the character Olivia Burke in six episodes of the TV series Gossip Girl in 2009.
Her older sister is actress Haylie Duff. She married Mike Comrie in February of 2010, and the couple had a son named Luca in March of 2012. The pair split up in January 2014 and their divorce was finalized in 2016. She later began dating Matthew Koma, with whom she welcomed a daughter named Banks Violet Bair in October 2018. She and Koma got married on December 21, 2019. In October 2020, the couple revealed that they're expecting their second child together.