Frei was born in Santiago to Eduardo Frei Montalva and María Ruiz-Tagle Jiménez. He received all his schooling at the Luis Campino Institute. He then attended the University of Chile, where he graduated as a Civil Engineer, specializing in hydraulics. After graduation, he followed advanced courses in management in Italy.
Frei took his first steps in politics while at the university, where he was a student leader. In 1958, he joined the Christian Democrat party, and in 1964 participated actively in his father's successful presidential campaign. Between 1969 and 1988 he concentrated on his profession, as one of the partners of Sigdo Koppers S.A., the largest engineering company in Chile.
In 1988 Frei founded and promoted the Comité Pro Elecciones Libres ("Committee for the promotion of free elections"). In 1989 he was elected Senator for Santiago, obtaining the highest amount of votes in the whole country. In the Senate, he presided over the Treasury and Budget commission, and was a member of the Housing commission.
In 1992 Frei participated in the primary presidential elections of his coalition, where he defeated Ricardo Lagos, and then went on to win the presidential elections in 1993. He took office on March 11, 1994 and was succeeded by Lagos in 2000. Frei assumed then, as a former President, a seat as senator-for-life in Congress. Given that Constitutional reforms in 2005 abolished life senators starting in 2006, Frei ran for an elected Senate seat in the December 2005 parliamentary elections and was victorious. He was elected in the electoral district of Valdivia Province and Osorno Province together with Andrés Allamand. On March 11 2006 Frei became President of the Senate, like his father, who was also President of the Senate after being President of the Republic.