Walter Thomas Mills was born in Duane, New York, USA, on 11 May 1856, the son of Charles Mills, a farmer, and his wife, Mahetabel Ladd, both Quakers. Nothing is known of Mills's early life. In 1868 he moved with his parents to Iowa and worked at a variety of jobs while saving the money to put himself through college. He graduated from both the College of Wooster (MA, in 1888) and Oberlin College, imbibing the principles of the social gospel. While at Wooster, Mills married a fellow student, Mary Wooster, whom he later divorced.
After leaving college Mills became an activist on behalf of prohibition. He wrote a textbook for prohibitionists on how to mobilise political pressure and an exposition of a co-operative industrial commune, and edited a journal. By the late 1880s he had broadened his interests and become a socialist. In the early 1890s he was in Chicago where, in 1901, he helped found the Socialist Party of America. He edited a socialist journal and wrote influential books on his evolutionist and ethical approach to politics, notably The struggle for existence (1904).
Mills engaged in several unsuccessful ventures in Chicago, tried to organise an agricultural commune, and organised and lectured at a 'People's University'. He became one of the leading Midwestern social evangelists, renowned for his ability to attract, entertain and instruct enormous audiences. Mills realised, however, that in the agrarian Midwest the working class alone could not bring about socialism; they would have to enlist the support of other groups, particularly small farmers. Increasingly, he devoted his ability and energy to the task of fighting off the revolutionaries, whether De Leonites or Industrial Workers of the World. He married Hilda F. Volck at Chicago on 17 October 1898; they were to have at least one son and one daughter.
Mills's reputation led to his touring Australia in 1911. While there he was invited to New Zealand by the Trades and Labour Councils of New Zealand and offered the challenging job of unifying the country's factious labour movement behind an organisation less militant and less exclusive than the New Zealand Federation of Labour. Mills accepted, and received an enthusiastic reception from Auckland's lively socialists and their champion, Harry Scott Bennett.
Mills stood four feet six inches tall, but his size belied his energy. His superb oratory and formidable grasp of socialist theory allowed him to woo the FOL rank and file. But he had wider appeal. He was a 'quaint little figure' with 'a grey leonine head, startling changes of expression in the magnetic eyes, wonderful modulations in the clear American voice, a penetrating, clutching, arresting influence, a torrent of Socialist oratory'. He was criticised by militants such as Pat Hickey, Paddy Webb and Bob Semple for kowtowing to the 'wowsers', and because he consorted with parsons. During the 1911 election campaign Mills faced Scott Bennett in a series of debates which attracted enormous interest and set out the ideological issues dividing the New Zealand labour movement.
The founding of the United Labour Party of New Zealand (ULP) in Easter 1912 by Mills and other moderates sharpened the conflict further. Mills and his followers hurled themselves into recruiting; the bitterness in Auckland occasionally exploded into fist fights. Mills travelled thousands of miles, speaking in almost every public hall in the country. He persuaded a great number of daily newspapers to publish a weekly column setting out the party's views.
Mills armed the supporters of the trades and labour councils with a new confidence about labour's role in guiding New Zealand's political evolution towards socialism; he also provided them with a reasoned defence of moderation against militancy. Whereas the councils had been anxious to maintain union control of any labour party, Mills persuaded them to think more broadly. In seeking to unite the labour movement he appealed to small farmers, housewives, prohibitionists, women and the urban middle classes. He advocated a tax on the unimproved value of land, the establishment of a state bank, a state-owned import–export agency to eliminate middle-men and stabilise markets, and a land lease based on occupation and tenure.
Mills slowly became disillusioned with the trades councils. Some refused to affiliate to the ULP but retained the affiliation fees paid by various unions; they failed to pay him regularly; and during the Waihi strike in 1912 some of their leaders allowed their enmity to the FOL 'Red Feds' to place them in a politically impossible situation. The failure of this strike persuaded the Red Feds to call a conference in 1913 to mobilise the labour movement against the Massey government. Delegates demanded that the ULP be invited and, despite the anger of some, Mills was accepted as a delegate. He persuaded the various factions at the conference to merge into two new organisations: the United Federation of Labour and the Social Democratic Party. Because of the paramount need for unity, many of his old enemies joined him in repudiating an exclusive reliance on industrial militancy.
In 1914 Mills and his wife left New Zealand as suddenly as they had come. Personal reasons may have persuaded them (Mills had a formidable reputation for philandering), but the outbreak of the First World War in August 1914 was probably more important. He disliked all empires, including Britain's. Back in the United States Mills became president of the International School of Social Economy which he had founded in 1900, supported the establishment of co-operative colonies, and joined the Socialist Party of America in its long fight to keep the United States out of the war. He did not keep in touch with anyone in New Zealand and publicly referred to the country only once, when he claimed that it was 'stolen from Holland'. Walter and Hilda Mills divorced at an unknown date. Mills married Maude E. Worley, formerly Evans, at San Jose, California, on 27 March 1931. Mills remained active on the American left until his death in Los Angeles on 7 May 1942. His wife and his son survived him.
Walter Mills is of considerable significance in the events before 1914 which began the unification of the New Zealand labour movement. His work in bringing together militants and moderates, and in committing the movement to evolutionary rather than revolutionary socialism, was crucial to its development as a political force.