He was elected Federal MP for Mtoko in 1957 under the Dominion Party banner. The Federation Minister of Justice, Julian Greenfield, found him 'somewhat impulsive and opinionated but entirely straightforward'.
When the Rhodesian Front was formed in early 1962 by Ian Smith and 'Boss' Lilford, a very wealthy and Right wing tobacco farmer they needed an Establishment figurehead. Field was chosen. He was a solid, trustworthy figure and no racist, even though "nearly everyone else in the new party was to the right of him". His wife said "he didn't really want to take it on, he wasn't really a political animal".
The "imperious and intolerant" (Godwin & Hancock, 1993) Field was elected, to his and many others surprise, as Rhodesia's first Rhodesian Front Prime Minister in the 1962 general election and served until he was replaced by Ian Smith in 1964. Field lent an air of respectability to the Rhodesian Front government, though his Cabinet was derided by one newspaper as "by no means an inspiring list". At the time of Field's election it was assumed that Britain would delay the process of independence for Rhodesia until "an African majority assumed power in Salisbury" (Godwin & Hancock, 1993). Many in the Rhodesian Front felt that Field did not fight hard enough for independence, in particular that the British had hoodwinked him on visits to London in June 1963 and January 1964 over promises of independence. His relatively short time in office saw the dissolution of the Central African Federation on 31 December 1963 though he did win the majority of the Federation's military and other assets for Southern Rhodesia.
His Cabinet included John Gaunt a former Federal MP for Lusaka and a former District Commissioner in Northern Rhodesia. Aware of discontent in Cabinet fomented by Gaunt, Field demanded his resignation in the spring of 1964. Gaunt asked him to wait over the weekend whilst he cleared up some matters in his office. In that time Gaunt and Ian Smith organised a plot against Field, now seen as ineffectual after his failure to win independence. Ken Flower, head of Rhodesia's Central Intelligence Organisation, an organisation Field had ordered be set up, had in fact warned him sometime previously there was a conspiracy against him, involving several of his ministers.
The caucus of the Rhodesian Front decided to ask for his resignation on 2 April 1964 and the decision was conveyed to Field the next day, though the formal demand was not made until a Cabinet meeting a few days later. Field was replaced as leader of the Rhodesian Front and as Prime Minister by Smith on 14 April 1964, despite the Governor Sir Humphrey Gibbs urging him to fight the rebels against him in his party.
He died in Rhodesia in 1969. His son attended Falcon College.