James Mitchell Ellis was born on 20 October 1865 at Banff, Banffshire, Scotland, the son of Elizabeth Ellis, a domestic servant. Elizabeth and her son subsequently emigrated to Otago, New Zealand, and on 5 September 1871 she married Peter Garrow, a farm servant, at Dunedin; he died the following year, by which time James had taken Garrow as his surname. A half-brother, William, was born in 1875. On 21 November 1877 Elizabeth Garrow married John Campbell Grant, a maltster, at Dunedin; they had one daughter, Elizabeth.
James commenced his working life as a pupil-teacher at Albany Street School, Dunedin. From 1886 to 1899 he taught at several Dunedin schools, studying for his BA part time at the University of Otago. He then worked as a real estate agent and sharebroker in the firm of Garrow and Stewart before becoming, in 1901, the first secretary of the Otago Employers' Association. The association had become moribund following the collapse of the 1890 maritime strike, but there was a need for employers to argue cases before the Court of Arbitration. In 1901 the organisation reconstituted itself with Garrow as full-time paid secretary. As part of his duties, he acted for trade organisations involved in disputes.
James Garrow appears to have taken the position more to advance himself and his career rather than from any particular commitment to the objectives of the association. He was nevertheless a competent and conscientious secretary, and described himself during this time as 'nothing if not methodical'. In 1904 he left the association and took the position of registrar of the University of Otago. He simultaneously studied for his LLB, graduating in 1905.
By 1908 Garrow was in private practice as a barrister and solicitor, but that year he was to begin university lecturing. The University of Otago had struggled to establish law teaching on a permanent basis, but the lectures offered in constitutional law and jurisprudence were terminated in 1902 and the School of Law was closed. Teaching in constitutional law and jurisprudence was resumed in 1905, but there was still no tuition offered in professional subjects until, as a result of pressure by the Law Students' Society, Garrow obtained the position of lecturer in law.
Garrow retained his position only until 1911, when he was appointed to the chair of English and New Zealand law at Victoria College, Wellington. Legal education at Victoria expanded greatly after Garrow's appointment, and he had an enormous teaching and administrative workload. The range of his teaching in some years included contracts, torts, property, evidence and procedure.
Garrow found that law teaching in New Zealand suffered from a severe shortage of New Zealand textbooks: the older English texts were becoming less and less useful as New Zealand law began to change. To meet this need Garrow began to prepare elaborate sets of lecture notes for circulation to his students. From these grew his sequence of textbooks spanning virtually the entire legal spectrum. In 1913 he published a textbook on property; this was followed by an annotated edition of the Crimes Act published in 1914, and works on the law of trusts and trustees (1919) and on evidence (1920). His last work was The law of wills and administration and succession on intestacy (1932).
Garrow's textbooks have gone through many editions and are still in use today. His approach to the law seems to have been heavily influenced by the positivist jurisprudence of the time, and his books are directed at the practical requirements of law students and practitioners, with very detailed notes and references. While his work strikes modern readers as meticulous but rather narrowly focused, his books were an enormous achievement and answered a desperate need for texts dealing with the rapidly evolving statutory and case law of New Zealand.
Garrow was long remembered as a kindly and considerate teacher. During the First World War he made strenuous efforts to correspond with Victoria students serving overseas. He led a quiet and retiring life, devoting himself to teaching and writing. His recreations included bridge, chess, music and gardening. He had married Bella Watters at Green Island, near Dunedin, on 15 April 1911; they had no children. After his retirement from Victoria University College in 1928 the Garrows moved to Nelson, and it was in retirement that he wrote his book on wills and administration. After a long period of poor health, possibly caused by his heavy workload of teaching administration and writing, Garrow died at Nelson on 7 October 1935 survived by his wife; he was buried at Karori. Colleagues remembered him as 'kindness personified'; he left behind him his books as 'monuments to his prodigious industry and encyclopaedic knowledge of the law'.