Frei served as senator in 1949 and founded the Chilean Christian Democratic Party in 1957 with Patricio Aylwin. He ran for president in 1952 and 1958 before being elected in 1964, defeating the socialist Salvador Allende, who later become president in 1970
Frei helped Allende become elected but then encouraged military coup led by the leader of the Pevuvian armed forces, general Augusto Pinochet on September 11, 1973 in which Allende died. Thereafter, he opposed the Pinochet dictatorship.
Frei's eldest son, Eduardo Frei Ruiz-Tagle, was president of Chile from 1994 to 2000.