Robert Brown, also known as Robert Brown tertius, was born in Glasgow, Scotland, probably between 1821 and 1824, the son of Mary Miller and her husband, George Brown, a bootmaker. Nothing is known of his early life or education. On 7 May 1849 he married Helen Nicolson at Edinburgh; they were to have at least three children. It is not known what became of Helen Brown, but Robert married Harriet Davis at Glasgow probably between 1872 and 1874. There seem to have been no children of this marriage.
By profession a shoemaker, Brown early developed an interest in natural history, particularly botany. From Glasgow he made long excursions on foot into the Highlands each year investigating the flora. Described as 'a poor man, uneducated as judged by ordinary standards', he nevertheless studied for a period at the Andersonian Institution in Glasgow under Professor Roger Hennedy.
With his second wife and family Brown emigrated to New Zealand about 1876. He bought a property in Christchurch, where he developed 'a beautiful garden filled with out-of-the-way exotic and indigenous plants.' For a time he worked as a shoemaker, but botany was his abiding interest. The ferns and mosses of Banks Peninsula first drew his attention and later he travelled widely, particularly in the South Island, collecting mosses and other plants and noting their distribution and habitats.
Brown was elected a member of the Philosophical Institute of Canterbury on 7 July 1887, although it was not until 1891 that he began to study mosses in earnest. In his first published paper on the subject he emphasised his reluctance to publish prematurely, but he feared that his specimens and camera lucida drawings deposited in the Canterbury Museum were in danger of being used by other workers. Although handicapped by primitive equipment, lack of access to contemporary scientific literature and a complete lack of scientific training, Brown published 22 papers on mosses between 1892 and 1902.
Many of the names Brown gave to species are now considered synonyms, and the value of his published work has been questioned. Nevertheless, his extensive collections, including mosses from habitats now destroyed, are still valuable. He is said to have been the first person to provide ecological information about mosses in New Zealand and to relate changes in plant form to ancient climatic changes in New Zealand. A remnant of Brown's herbarium is now housed at Landcare Research New Zealand (formerly the Botany Institute) at Lincoln, the remainder being incorporated in the herbarium of the British Museum.
Spare of body and physically weak in later years, Brown could still walk 30 and even 40 miles in one day, carrying a heavy burden and sleeping rough if need be. He was one of the first to cross Clinton Saddle to Milford Sound. He ascended the Thomson Ridge in Stewart Island in his late 70s, and when over 80 years of age walked, botanising all the way, from Kaikoura to Blenheim. Even in old age his figure was erect and energetic and his 'rugged, intellectual face' crowned by a wealth of snow-white hair.
Robert Brown died at his home in Christchurch on 13 December 1906, survived by two sons and one daughter. Harriet Brown had died in 1886. Brown's botanical views and practices had influenced others in the field, and he was known for his saying (perhaps a fortunate consequence of his lack of formal training): 'Heed not what books or authority teaches but, in order to really learn, go to the plants themselves.'