Horgan has appeared on stage, television and screen. Her first credited appearances on television were in "The State We're In" (2002) and "Monkey Dust" (2003), two sketch shows based on news and current affairs. She also contributed material to "Monkey Dust". Her first named acting role on television was as Theresa O'Leary in "Absolute Power" (2003), a comedy set in the world of public relations and starring Stephen Fry. In 2005 she made her big-screen debut as Beth in "Imagine Me & You", a British-American romantic comedy directed by Ol Parker.