He is the son of Toralf Westermoen, who among other things is a former member of the national Parliament. Thore Westermoen himself served as a deputy representative to the Parliament of Norway from Vest-Agder during the terms 1989–1993, 1997–2001 and 2005–2009.
Following the 1999 election, Westermoen became the new county mayor (fylkesordfører). He was re-elected in 2003 and 2007. After the 2011 Norwegian local elections the local alliance he led was dissolved. He was offered the county mayor post if the Christian Democratics cooperated with Labour, but they rejected. Westermoen was relegated to deputy county mayor.