Harold Montague Rushworth was born at Croydon, Surrey, England, on 18 August 1880, the son of Gertrude Wheeler and her husband, William Rushworth, an architect. He was educated at Rugby School and Jesus College, University of Oxford. Although he studied law and was called to the Bar he did not practise, instead becoming a civil engineer and surveyor. He worked for the London County Council from 1905 to 1914 and became manager of its Properties Division. On 24 August 1905 at Streatham, Surrey, he married Maud Rose Sylvia Hazel.
In 1902–3, towards the end of the South African War, Rushworth had served in the London Scottish Regiment. When the First World War began he joined the 7th City of London Regiment, arriving on the western front in February 1915. Two months later, while leading a patrol in no man's land, he was seriously wounded. He recovered and rejoined his regiment, but in September, at Loos-en-Gohelle, his right knee was shot away. In 1916–17, while attached to the Royal Engineers, he learned to fly and joined the Royal Flying Corps. In August 1917 he was shot down over Passchendaele (Passendale) and captured. After three months he was released and repatriated to London.
In 1919 Rushworth returned to work at the London County Council, but in 1923 he emigrated with his family to New Zealand. He commenced farming at Opua in the Bay of Islands, and became a member and Auckland sub-provincial secretary of the New Zealand Farmers' Union. He served on the union's national executive from 1928 to 1931, and from 1935 to 1944. He soon associated himself with those who believed that the Reform government had become dominated by its urban wing and was neglecting farmers, especially smaller dairy farmers in the north. Many not only wanted an independent Country Party to look after their interests, but were also strongly influenced by Major C. H. Douglas's monetary theories, which became known as Douglas Credit or Social Credit. Their author was, like Rushworth, an Oxbridge-educated engineer and former Royal Flying Corps officer, but there is no evidence that the two men knew each other before Douglas visited New Zealand on a lecture tour in 1934.
Together with the formidable Colonel S. J. E. Closey, Harold Rushworth became one of the leaders of the New Zealand Farmers' Union (Auckland Province), the organisation most influenced by social credit ideas. In 1928 he was elected Country Party MP for the Bay of Islands in one of the closest and most acrimonious contests in New Zealand's electoral history. Rushworth appeared to have won by one vote, but a recount tied the result and the returning officer gave his casting vote to the incumbent Reform MP, Allen Bell. A magisterial recount then awarded the seat to Rushworth by two votes, but an electoral petition forced a by-election, which Rushworth won by 474.
Described as 'the cinema ideal of an English gentleman', who 'with his impeccable Oxbridge voice and manner could deeply impress an audience', Rushworth held political views that are difficult to categorise. In England he had moved from support for the Conservatives towards the Labour Party, becoming friendly with the prominent Labour politician Arthur Henderson. In New Zealand he was widely seen as an ally of the Labour Party, which shared his suspicion of bankers and the conservative press. Rushworth and Closey co-operated with Michael Joseph Savage from 1932 and Labour did not stand a candidate against him in 1935. Indeed, Hikurangi coalminers and Okaihau railway workers voted overwhelmingly for Rushworth and were decisive in his 1928 victory. Although he became president of the Douglas Social Credit movement, he claimed that his ideas on political economy owed more to Adam Smith than Douglas.
An MP for 10 years until his retirement in 1938, Rushworth was not a particularly dynamic or effective politician. He was unable to develop a lasting Country Party or exert much influence on the direction of government. He admitted that he was not ruthless enough for the rough-and-tumble of politics and preferred 'a good book by a fireplace'.
Harold Rushworth died at Castor Bay, Auckland, on 25 April 1950, a year after he had been elected a life member of Federated Farmers of New Zealand. He was survived by his wife, two daughters and a son. The social credit tradition which he had established in Northland laid the basis for the election in 1966 of the New Zealand Social Credit Political League's Vernon Cracknell as MP for Hobson, a seat that incorporated most of Rushworth's old Bay of Islands electorate.