Alfred Humphrey Hindmarsh was born at Port Elliot, South Australia, on 18 April 1860, the son of John Hindmarsh and his wife, Mary Long. Alfred's grandfather, Sir John Hindmarsh, had been the first governor of South Australia. Unable to work successfully with the other officers of the colony, he had been recalled to England in 1838. Alfred's father was called to the Bar in London and returned to South Australia in 1855. When Alfred was 10 his mother died; his father subsequently remarried. The family moved to New Zealand in 1878, settling in Napier where John Hindmarsh practised as a barrister and held a large sheep run.
Alfred Hindmarsh was educated at St Peter's College, Adelaide, and trained as a lawyer in a Dunedin law firm. In 1891 he was admitted as a barrister and solicitor of the Supreme Court of New Zealand in Christchurch. Shortly afterwards he moved to Wellington where, on 3 October 1892, he married Winifred Taylor.
From the mid 1890s Hindmarsh was active in the Wellington labour movement. He served as secretary of the Wellington Socialist Education League – although his politics could not be described as socialist – and was prominent in the local eight-hour movement. Later, in 1915, he was foundation president of the Workers' Educational Association in Wellington. He was a relatively minor figure in the Wellington union movement but was president of the local branch of the Federated Cooks and Stewards' Union of New Zealand from about 1894 and of the Wellington Match-factory Employees' Union from 1913, holding both positions at the time of his death in 1918.
More controversially, Hindmarsh was president of the Wellington branch of the Federated Seamen's Union of New Zealand from 1895 until 1898 when intense infighting split the organisation. He and William Jones, secretary of the branch, were critical of the Liberal government, which put them at odds with the union's national leaders who were strong Liberal supporters. At a meeting held in February 1898 to select a labour candidate for the Wellington South by-election, Hindmarsh, who was seeking selection, stressed the need for an independent labour voice in Parliament. He accused the premier, R. J. Seddon, who was at the meeting, of dictatorial practice and of trying to make the labour movement a slave to the Liberals. He also suggested that the premier did little for working people in his capacity as minister of labour.
While this stance alienated Hindmarsh from the national leaders of the Federated Seamen's Union, it struck a chord with many delegates. Nevertheless, he missed selection and instead of supporting the candidate chosen by the Wellington Trades and Labour Council and endorsed by the Liberals, he and Jones supported an independent who was similarly critical of the Liberals. This action and an ensuing dispute over the use of union funds caused a split in the Seamen's Union. The bulk of the Wellington members sided with their branch officials and established a breakaway union, although Hindmarsh was only very briefly associated with it.
For the rest of his political career Hindmarsh advocated an independent voice for labour in Parliament. He made his initial mark in local body politics. After an unsuccessful attempt in 1901, he was elected to the Wellington City Council in 1905 on the ticket of the Independent Political Labour League. He had been a founding member of the league and remained on the council until standing down in 1915. From 1911 he was also a member of the Wellington Harbour Board.
In 1905 Hindmarsh stood unsuccessfully for the parliamentary seat of Newtown, as a candidate for the Independent Political Labour League. He was president of the league in 1906–7, and in 1911 won the Wellington South seat on the second ballot for the New Zealand Labour Party, which had taken over the mantle of the league. He was to represent the electorate until his death.
In 1912 Hindmarsh joined the United Labour Party of New Zealand, becoming an executive member. In 1913, along with others (including J. T. Paul, David McLaren and E. J. Carey), he opposed the agreement reached at the Unity Congress that led to the establishment of the Social Democratic Party. Hindmarsh never joined the SDP and in 1914 was re-elected to Parliament under the banner of the Wellington Labour Representation Committee. Although he was critical of the militant socialists, Hindmarsh did not court non-labour support to counter their influence. He refused, for example, to attend the Liberal caucus in Parliament as was the practice of some of his labour colleagues.
Hindmarsh made his mark in Parliament during the First World War when the small but disparate group of labour members formally organised themselves as a caucus and became the parliamentary opposition. Hindmarsh was elected caucus chairman in July 1915. He did not oppose the war; indeed, he had been a supporter of compulsory military training and conscription.
In July 1916 Hindmarsh attended the conference that established the second New Zealand Labour Party. He did not hold executive office in the organisation, but remained the leader of the Labour members in the House and as such was the first leader of the parliamentary wing of the New Zealand Labour Party. Later in the war he was increasingly out of step with his more militant colleagues, some of whom considered he was moving closer to the Liberal party.
Hindmarsh was considered pleasant and cultured and, despite a speech impediment, was a breezy and emphatic speaker. He certainly spoke his mind and was just as likely to criticise friends as opponents, although this was usually taken as constructive. He was generally admired for his consistency, honesty of purpose and sincerity. Alfred Hindmarsh died in Wellington on 13 November 1918, a victim of the influenza epidemic. He was survived by four children. Winifred Hindmarsh had died in 1916.