When Fianna Fáil won power in 1932, Lemass became minister for industry and commerce in De Valera's cabinet and remained in this position until 1939, when he became minister for supplies during World War II. Lemass's success at provisioning Ireland, which was a neutral power during the war, convinced De Valera to name him deputy prime minister in 1945. Fianna Fáil lost the 1948 election but won again in 1951. With De Valera again as prime minister, Lemass served once more as minister for industry and commerce during 1951–1954 and 1957–1959. On 23 June 1959, Lemass succeeded the retiring De Valera as prime minister and party leader.
The Lemass government's successful economic policies went along with continued progress in international affairs. Ireland was elected to the United Nations (UN) Security Council in 1962 and was visited by U.S. President John F. Kennedy in 1963. Moreover, Lemass's more moderate stance on the Irish partitioning issue enabled him to improve relations with Northern Ireland. He resigned as prime minister and party leader on 10 November 1966 but remained a member of parliament until 1969. Lemass died on 11 May 1971 in Dublin.
Farrell, Brian. Séan Lemass. Dublin: Gill and Macmillan, 1983.; Lee, Joseph J. Ireland 1912–1985: Politics and Society. Cambridge: Cambridge University Press, 1989.