He was originally a member of the Labour Party, but left in 1995 to jointly establish the United New Zealand party with six other sitting MPs. Matthewson then became United's leader. Although described as charismatic and telegenic, Matthewson was often perceived as an "intellectual lightweight", and United was criticised as lacking any concrete policies.

When the party formed a coalition with the governing National Party in 1996 he was made a Cabinet Minister.

In the 1996 election, United was almost completely wiped out, with only Peter Dunne keeping his seat. Matthewson did not return to Parliament.

Currently he is the Director of Development at University of Otago.

He holds a PhD in Civil Engineering.