In 1988 Rehn was elected as a city councillor in Helsinki. He was vice president of the Centre Party from 1988 to 1994, having been president of its youth wing from 1987. Elected to the Finnish Parliament in 1991, Rehn led the Finnish delegation to the Parliamentary Assembly of the Council of Europe, and was a special adviser to the Finnish Prime Minister Esko Aho from 1992 to 1993. He left the Finnish Parliament in 1995 to become an MEP, aligned to the liberal group. He was, however, not re-elected in the 1996 election.