Algernon Phillips Thomas (the name Withiel was added later) was born on 3 June 1857 at Birkenhead, Cheshire, England, the fifth of eight children of Edith Phillips and her husband, John Thomas, an accountant. Algernon had a grammar school education and then went to Balliol College, Oxford, on a scholarship, graduating BA in 1878 and MA in 1881. In 1879 he won a Burdett-Coutts University Scholarship and was appointed demonstrator at the University Museum under the distinguished biologist George Rolleston. At Rolleston's suggestion he undertook a study for the Royal Agricultural Society of the life cycle of the parasitic fluke which caused liver rot in sheep. His discovery in 1882 (shared with Rudolf Leuckart of Leipzig) that in its larval stage the fluke passed through an intermediate host (a snail) before infecting the sheep, was a finding of great scientific and economic importance.
Thomas was ambitious and might have gone on to further scientific success, but he was also ruled by financial prudence. Like other young scholars of promise but uncertain prospects in Britain, he turned to the colonies. Perhaps influenced by Rolleston, whose brother William had reached cabinet rank in New Zealand, Thomas applied for and was appointed to the professorship of natural science at the university college which was to be established in Auckland.
He arrived on 1 May 1883 and four weeks later delivered the opening lecture for Auckland University College. The college began modestly with makeshift rooms and just two other professors, F. D. Brown (chemistry) and T. G. Tucker (Classics and English). Because the fourth professor, G. F. Walker, had drowned immediately after arriving, Thomas temporarily added mathematics (which he shared with Professor Brown) to his natural science subjects. Thomas's position was renamed professor of biology and geology in 1885, and remained so until his retirement.
The professorial post, with its comfortable income (£700 per annum plus student fees), gave Thomas a position of some social standing in Auckland. He joined the Northern Club, the Auckland Society of Arts and the Auckland Institute and Museum; he had his own pew at church, took dancing lessons and played tennis. He courted Emily Sarah Nolan Russell, daughter of Auckland solicitor John Benjamin Russell and his wife, Mary Nolan, and niece of the renowned financier Thomas Russell. The couple were married at a fashionable wedding at St Andrew's Church, Epsom, on 19 November 1887; his mother-in-law's wedding present included a parcel of goldmining shares. There were three sons and one daughter of the marriage. Emily Thomas became known as a woman of advanced ideas. She joined her husband on his students' excursions and bicycle tours. Sadly, in 1908 she was admitted to Ashburn Hall private asylum in Dunedin. She remained there until March 1950 when she was transferred to Kingseat Hospital near Auckland. She died at Kingseat on 6 July the same year.
As well as carrying out his formal teaching duties, Algernon Thomas became a perennial public lecturer on scientific and technical subjects and a vigorous advocate of practical education. As an accepted expert – one of the few outside the Wellington scientific establishment under James Hector, director of the Geological Survey and Colonial Museum – he was often called on to make reports or give advice on geological, biological and bacteriological matters. He also carried on some research, most notably on various New Zealand 'living fossils': the primitive plants Phylloglossum and Tmesipteris, and the tuatara. His papers on these topics of general scientific interest were published in the Proceedings of the Royal Society of London; lesser material appeared in the Transactions and Proceedings of the New Zealand Institute. He joined T. J. Parker, his counterpart at the University of Otago, on expeditions to obtain tuatara for study and in 1885 supported Parker's suggestion that the species should be given some protection. The matter was referred to Hector, who acidly enquired how many specimens Thomas and Parker had taken. Thomas subsequently advocated other measures for the preservation of native flora and fauna, with greater success.
In 1913, after the college made changes to the terms of his employment, Thomas retired from his professorial post. His investments had brought him comfortable wealth; and after returning briefly to England he concentrated on his garden, where he delighted in producing Auckland's earliest and best daffodils, and on his work for numerous educational bodies. He was a long-serving member of the senate of the University of New Zealand and the Auckland University College council. As chairman of the Auckland Grammar School Board he presided over the growth and development of the grammar schools. For generations of Auckland students the patriarchal figure of Thomas, with his eagle eye, Dundreary whiskers and inexhaustible stores of wisdom, became an educational institution. In 1937 he was appointed a KCMG. He died at Auckland, two weeks after his investiture, on 28 December that year.