In 2005, MacArthur co-wrote, produced and starred in a 16-minute short film, The Adventures of Big Handsome Guy and His Little Friend, with Jason Winer. The short was produced as a pilot for TV in 2006. He also appeared as a waiter in the episode of Curb Your Enthusiasm where everyone chooses Larry's wife over him after the break-up. In 2007, MacArthur appeared with fiancé Ali Larter in National Lampoon's Homo Erectus, and also appeared in a comedic supporting role in the Disney movie The Game Plan. Along with recurring roles on the television shows How I Met Your Mother and Worst Week, he appeared in supporting roles in the 2010 films She's Out of My League and Life as We Know It.