MacArthur completed his high school studies at Deerfield Academy. After this, he joined Bowdoin College to pursue a Bachelor’s in Government. To pursue his interest in acting, he later enrolled in Groundlings Theater Company and Atlantic Theater Company, too.

He made his feature film debut with Homo Erectus, portraying Thudnik. However, he is most widely known for playing Jay Geils in Angie Tribeca. His other acting credits also include Go On, Worst Week, A Haunted House 2, Being Frank, and others. He also made guest appearances in popular shows like How I Met Your Mother and Happy Endings.