John Carruthers was born at Inverness, Inverness-shire, Scotland, on 21 June 1836, the son of Janet Roberts and her husband, Robert Carruthers, editor and proprietor of the Inverness Courier and author of works on Alexander Pope. John was educated at Christ's Hospital, London, and at the Grammar School and Royal Academy of Inverness. His father intended him to go to the University of Cambridge; instead, he travelled to Canada and began an engineering career on the Great Western railway. He subsequently worked on railways in the United States, Russia and Mauritius. In 1866 he was elected an associate of the Institution of Civil Engineers, and was later a member.
On 27 February 1866 at Burnfoot, near Inverness, John Carruthers married Susan Elizabeth Davidson, whose grandfather, father and brother were engineers. A son and a daughter were born to the couple. After his marriage Carruthers spent a short time in Egypt and was then engaged in irrigation canal design and construction in the Madras Presidency in India. He had returned to England by 1871.
That year Julius Vogel travelled to England to raise a large loan for capital works in New Zealand. Learning of Carruthers's extensive railway construction experience in various countries, Vogel appointed him engineer-in-chief for New Zealand, with particular responsibility for railway construction. John Blackett, who had been acting engineer-in-chief, became assistant engineer-in-chief and was put in charge of road construction – the other major engineering initiative of the 1870s. Carruthers arrived in 1871 and took up office in June.
Over the next seven years John Carruthers was responsible for the survey of over 1,000 miles of railway line and the construction of track, bridges, tunnels and accessory buildings throughout the country. In addition he had the task of training a largely inexperienced staff. As if this were not enough, he was expected to oversee the construction of major harbour developments, water race canals and other engineering works. The supervision of these various projects was difficult as communication with Wellington from some sites took several weeks. Carruthers's first report to Parliament in 1871 has an air of breathlessness which suggests the speed at which he had familiarised himself with the work under his command.
Carruthers had an innovative approach to engineering problems. He supervised the survey and design of one of the most remarkable engineering achievements in New Zealand, the Remutaka incline. Carruthers made the decision that an incline system should be used, and he chose the Fell centre-rail system to combat the problem of steep grades. He later recommended the use of an incline on the Puerto Cabello and Valencia railway, Venezuela, and reported on the reasons for his choice in a brief paper to the Institution of Civil Engineers in 1889. This paper drew the attention of other engineers to the Remutaka Incline and sparked a lively debate, generally favourable to Carruthers. Perhaps his next most notable work in New Zealand was the Rakaia Gorge bridge, a unique piece of invention that was still in use 100 years later.
In 1878, as the result of political manoeuvres, the Public Works Department was restructured and Carruthers was offered a lesser position in charge of North Island engineering. He declined it and returned to England in 1879 to practise on his own account. From his base in London Carruthers supervised work in Venezuela and Argentina, and was consulting engineer to the government of Western Australia. From 1903 he practised in conjunction with J. D. Elliot, and his own son F. Gilbert Carruthers. However, he retained his link with New Zealand. He was a consulting engineer for the New Zealand government until his death, and revisited New Zealand in 1907.
Carruthers was widely travelled and had the attributes of a man of action. He was also reflective and fond of intellectual discussion. He had the habit of rising in the middle of the night and going for long walks during which he would ponder the issues that interested him. Engineering matters were by no means his only concern. While in New Zealand he read a paper on economic reform to the New Zealand Institute. In his address he confidently challenged the theories of John Stuart Mill. On his return to London in 1879 he was active in socialist groups, including the Hammersmith Socialist Society, and published several works on economics. He was at this time a close friend and associate of William Morris.
John Carruthers died on 2 September 1914 in London. His ashes were buried at Tomnahurich, Inverness. It is not known when or where Susan Carruthers died.