Adam Hamilton was born at Forest Hill, Southland, on 20 August 1880, the son of Scottish parents John Hamilton, a farmer, and his wife, Mary McIlwrick. He attended school at Forest Hill and Lochiel, leaving from standard six to work on his father's farm. At 25 he enrolled at Dunedin Technical School; he may also have gone to classes at the University of Otago. He spent from 1909 to 1912 at Knox College training for the Presbyterian ministry, but on graduation decided not to enter the church.
On 26 March 1913, in Hokonui, Hamilton married Mary Ann McDonald. The following year, with his brother John Ronald Hamilton, he started a firm of grain merchants in Winton. He was rejected for military service during the First World War because of a goitre condition, but did work for the YMCA providing support and recreation for soldiers.
In 1919 both brothers were elected to Parliament as Reform Party candidates; Adam in the Wallace electorate and John in Awarua, where he defeated the long-serving Liberal member and former prime minister Sir Joseph Ward. Following their election the Hamiltons sold their company to the Southland Farmers' Co-operative Association. Adam Hamilton remained closely associated with the firm and in 1936 became chairman of its board of directors. He also served from 1922 to 1931 as producers' representative on the New Zealand Meat Producers Board and from 1928 to 1931 as chairman of the Farmers' Co-operative Wholesale Federation. He was on the Southland Electric Power Board from 1922 to 1925 and a trustee of the Invercargill Savings Bank for 29 years.
At the 1922 election both Adam and John Hamilton lost their seats in Parliament. However, the 1925 election was a resounding success for the Reform Party under its new leader, Gordon Coates, and the Hamiltons regained their seats. Although John was again defeated in Awarua in 1928, Adam was to hold Wallace for the Reform Party, then coalition Reform, and finally for the New Zealand National Party until he retired in 1946.
He joined the coalition government formed in 1931 under the leadership of George Forbes (United) and Coates (Reform) to try to combat the depression. Hamilton was minister of internal affairs, and postmaster general and minister of telegraphs. He also held the difficult and unpopular posts of minister of labour (1931–35) and minister in charge of employment (1933–34), receiving considerable opprobrium for the suffering caused by the lack of jobs and the relief schemes and camps into which many unemployed men were recruited.
Somewhat more satisfying was his development of a national broadcasting service. At the 1935 election, however, senior civil servants in the Post and Telegraph Department jammed a pro-Labour broadcast by Colin Scrimgeour on the private Auckland radio station 1ZB. They subsequently tried to cover up the incident and Hamilton denied prior knowledge, but his reputation was tarnished.
On 31 October 1936 he was chosen as leader of the opposition and of the new National Party, set up by the defeated remnants of the Reform, United and Democrat parties. He gained the post primarily because Coates and Forbes detested each other, and neither would serve under the other's leadership. Forbes preferred Charles Wilkinson, an independent member, but Coates and a group of Reform members threatened to leave the National Party and re-establish the Reform Party unless Hamilton was successful. He was chosen by one vote.
Hamilton was a tall, rugged man with a quiet personality, sincere, dependable and experienced, but he lacked charisma and was too closely identified in the public's memory with the conservative government during the depression. He never really established himself as a leader in his own right and was always seen as a lieutenant of Coates, and even as a 'seat-warmer' for his previous leader. Many influential people in the caucus and party organisation failed to give Hamilton the support a party leader was entitled to expect.
He also had the misfortune to be continually compared with Michael Joseph Savage at the peak of the Labour leader's popularity. Nor was Hamilton able to shake Savage by moving a vote of no confidence in the Labour government for – among other things – voting in a minority of two with the Soviet Union in the League of Nations against the Italian conquest of Abyssinia (Ethiopia). Hamilton believed Labour's action was 'a direct threat to the solidarity of the Empire'.
At the 1938 election Hamilton and National fought a negative campaign. They were convinced that all they had to do to win was to organise, criticise, publicise and prevent the splitting of the anti-Labour vote. Hamilton himself felt that more elections were won by criticism of a government than by policy, but the campaign, which accused Savage and Labour of trying to establish a communist dictatorship in New Zealand, went too far and was clearly counter-productive. His own lacklustre performance and National's overwhelming defeat weakened Hamilton's always tenuous hold on the National Party leadership.
The outbreak of the Second World War diverted attention from domestic politics, and in May 1940 Hamilton proposed the formation of a bipartisan wartime coalition government. Although the new Labour prime minister, Peter Fraser, refused, he agreed to set up a six-person war cabinet, made up of four Labour and two National MPs, to look after the war effort while the Labour cabinet governed the country. Hamilton and Coates joined the War Cabinet on 16 July, with Hamilton as minister in charge of war expenditure. However, his opponents in the National Party argued that a minister could not be leader of the opposition and Sidney Holland, supported by Forbes, mounted a leadership challenge. On 25 November, by a caucus vote of 13 to 8, Holland replaced Hamilton as leader.
Hamilton continued to serve in the War Cabinet, and after Japan entered the war in 1942 he and Holland also became members of an expanded War Administration of seven Labour and six National members. Holland became a seventh member of the inner War Cabinet. A few months later, however, Holland and the National Party withdrew from both cabinet and administration in protest at Labour's handling of a miners' strike near Huntly. Although Hamilton and Coates resigned with their colleagues they issued a public statement critical of their leader's and party's action and withdrew also from the National caucus. They then rejoined the War Cabinet, where Hamilton continued to serve until 1945.
The death of Coates shortly before the 1943 election isolated Hamilton, who returned to the National Party and stood again as a National candidate. He did not stand at the 1946 election, and died in Invercargill on 29 April 1952. He was survived by his wife: there were no children of the marriage. Outside his political and business life Hamilton was interested in shooting and fishing. His lifelong commitment to the Presbyterian church and to Knox College was indicated by a bequest of over £9,000.