Massey was born into a farming family, and grew up near the city of Derry in Northern Ireland). He was born in 1856. His family moved to New Zealand in 1869, although Bill Massey himself remained in Ireland for a further year to complete his education. After arriving in New Zealand, Massey worked as a farmhand for some years before acquiring his own farm in 1877. Five years later, Massey married his neighbour's daughter, Christina Allen.
Massey gradually became more prominent in his community. This was partly due to his involvement the school board, the debating society, and freemasonry, but the most important groups he participated in were farming associations. Because of his prominence in these circles, he became involved in political debate, working on behalf of rural conservatives against the Liberal Party government of John Ballance.
In 1893, Massey stood as a candidate in parliamentary elections, but was unsuccessful, losing to the Liberal candidate. Early the following year, however, Massey was invited to contest a by-election in a neighbouring electorate, and was victorious.
Massey joined the ranks of the (mostly conservative) independent MPs opposing the Liberal Party (now led by Richard Seddon). These MPs, however, were poorly organized and dispirited, with little chance of unseating the Liberals. William Russell, official Leader of the Opposition, was able to command only fifteen votes. Massey brought increased vigour to the conservative faction.
While the conservatives did rally for a time, support for the Liberals increased markedly during the Boer War, leaving the conservatives devastated. Massey's political career, however, survived the period. Despite a challenge by William Herries, Massey remained the most prominent opponent to the Liberal Party.
After Seddon's death, the Liberals came to be led by Joseph Ward, who proved more vulnerable to Massey's attacks. In particular, Massey made gains by claiming that alleged corruption and cronyism within the civil service was ignored or abetted by the Liberal government. His conservative politics also benefited him when voters grew concerned about militant unionism and the supposed threat of socialism. In 1909, he announced the creation of the Reform Party, led by himself and backed by his conservative colleagues.
In the 1911 elections, the Reform Party managed to gain more seats than the Liberal Party, but did not gain an absolute majority. The Liberals, relying on support from independents who had not joined Reform, were able to stay in power until the following year, when they lost a vote of no confidence. Massey was sworn in as Prime Minister on 10 July.
As time passed, however, some members of Reform grew increasingly frustrated at Massey's dominance of the party. He also earned the enmity of many workers with his harsh response to miners' and waterfront strikes in 1912 and 1913. The use of force to deal with the strikers made Massey an object of hatred for the emerging left-wing. However, conservatives (many of whom believed that the unions were controlled by socialists and communists) generally supported Massey, saying that his methods were necessary.
The outbreak of the First World War, however, diverted attention from these matters. The 1914 election left Massey and his political opponents stalemated in parliament, with neither side possessing enough support to govern effectively. As such, Massey reluctantly invited Joseph Ward of the Liberals to form a war-time coalition (created in 1915). While Massey remained Prime Minister, Ward gained de-facto status as joint leader. Massey and Ward travelled to Britain several times, both during and after the war, to discuss military cooperation and peace settlements. During his first visit, Massey visited New Zealand troops, listening to their complaints sympathetically. This angered some officials, who believed that Massey undermine the military leadership by conceding (in contrast to the official line) that conditions for the troops were indeed unsatisfactory. The war did, however, reinforce Massey's strong belief in the British Empire and New Zealand's links with it.
The coalition government, partly because of the difficulty in obtaining enough consensus to implement meaningful policies, had grown increasingly unpopular by the end of the war. Massey was particularly worried by the rise of the Labour Party, which was growing increasingly influential. Massey also found himself fighting off criticism from within his own party, including charges that he was ignoring rural concerns. He dissolved the coalition in 1919, and fought both the Liberals and Labour on a platform of patriotism, stability, support for farmers, and a public works program. He successfully gained a working majority.
Economic problems, however, lessened support for Reform. In the 1922 elections, Massey lost his majority, and was forced to negotiate with independents to keep his government alive. He was also alarmed by the success of Labour, which was now only five seats behind the Liberals. He began to believe that the Liberals would eventually disappear, with their supporters being split between Reform and Labour - the socially liberal wing to Labour and the economically liberal wing to Reform. Massey set about trying to ensure that Reform's gain would be the greater.
In 1924, however, illness forced Massey to relinquish many of his official duties. The following year, he died of his illness. A memorial to him exists in New Zealand's capital city. Massey University is also named after him - the name was chosen because the university initially had a focus on agricultural science, matching Massey's own farming background.