Active in politics since the mid-1990s, Horne was elected member of the executive committee of Sola municipal council after the 1995 local elections, and was a member of the Rogaland county council from 1999 to 2005.
On 16 October 2013, after the defeat of the Red-Green Coalition and establishment of the Conservative-led Solberg's Cabinet, Horne was appointed Minister of Children of Equality, replacing Inga Marte Thorkildsen from the Socialist Left Party.