Sonia's political career started in 1993 when she joined the People's Party (Partido Popular). In 1999 she was elected a member of Alicante City Council. In 2003 she became the first deputy to the mayor.
On September 17, 2008, following the resignation of Luis Díaz Alperi, Sonia Castedo was elected Alicante mayor. She is the first woman ever to occupy this position.