At age 12, having left school after his father's death, Holyoake worked on the family hop farm near Nelson while his mother, Esther, continued his education at home. He became involved in Farming Groups which finally led to him representing Motueka in 1932. (He contended unsuccessfully for the seat in 1931 and then won it in a by-election one year later, becoming the youngest parliament member at the time.) He played a key role in establishing the National Party in 1936 but failed to win re-election in 1938.
In 1943 he returned to Parliament as MP for Pahiatua and became Deputy Leader of National in 1946. National gained a majority in the 1949 election and Holyoake became Minister of Agriculture and the first person to be formally appointed Deputy Prime Minister.
Holyoake became Prime Minister for a few weeks before the 1957 general election because the Prime Minister at the time (Sidney Holland) retired due to ill-health. He was Leader of the Opposition for three years before National returned to power in 1960.
Sir Keith was the third longest-serving Prime Minister (just under 12 years) that New Zealand had ever had (he's surpassed by Richard Seddon's 13 years as Premier/Prime Minister and William Massey's close to 13 years as Prime Minister) and was known for his diplomatic style and "plummy" voice. He was also fondly (or mockingly) known as Kiwi Keith. (a name given to him in childhood). In 1972 Holyoake resigned as Prime Minister to ease the succession for his deputy and friend, John Marshall. Upon Labour's defeat in the December 1975 general elections, he remained in the Cabinet as the first Minister of State until 1977 when he became Governor-General. He served as Governor-General until 1980.
Sir Keith died in December 1983, aged 79, in New Zealand.