In 2003, Hahn appeared in a supporting role alongside Kate Hudson and Matthew McConaughey in the romantic comedy film How to Lose a Guy in 10 Days. The following year she appeared in Win a Date with Tad Hamilton!, Anchorman: The Legend of Ron Burgundy with Will Ferrell and Paul Rudd, Around the Bend, and Wake Up, Ron Burgundy: The Lost Movie. She later had more supporting roles in films, including the 2005 romantic comedy-drama A Lot like Love starring Ashton Kutcher and Amanda Peet; The Holiday (2006) with Cameron Diaz and Kate Winslet; the science fiction adventure drama The Last Mimzy (2007) alongside Rainn Wilson; Step Brothers (2008) starring Will Ferrell and John C. Reilly; Revolutionary Road (2008) starring Leonardo DiCaprio and Kate Winslet; The Goods: Live Hard, Sell Hard (2009), How Do You Know (2010) with Reese Witherspoon, Owen Wilson, Paul Rudd, and Jack Nicholson, Our Idiot Brother (2011) with Paul Rudd, and Wanderlust (2012) with Paul Rudd and Jennifer Aniston.