Scottish surgeon and naturalist, who trained in both botany and medicine at Edinburgh University and was briefly employed as a Gardener at the Royal Botanic Garden, Edinburgh before travelling south to Caernarvon as an assistant to a surgeon. He joined the Royal Navy (1782) as an Assistant-surgeon and served in Nova Scotia, collecting plants locally whenever his duties allowed. Menzies was brought to the attention of Joseph Banks on whose recommendation he was appointed surgeon and naturalist to accompany Captain George Vancouver, who had sailed on both the second and third expeditions of James Cook, and was about to undertake a voyage around the world (1791-1795) in HMS Discovery. Menzies collected many thousands of specimens from around the Pacific, including Australia, New Zealand, Polynesia and the Pacific coast of what is now the United States and Canada. The relationship between Menzies and Vancouver was not always cordial, and Menzies sometimes collected in spite of Vancouver's wishes. On the return journey the Captain placed him under arrest for insolence and contempt; a month later Vancouver withdrew his charges and request for a court-martial after a formal apology from Menzies. Many of the earliest collections of plants from western North America were made by Menzies and his work shows meticulous attention to detail.

After returning from the expedition, Menzies served briefly with the Navy on HMS Sans Pareil in the West Indies. On retirement from the Navy, he became a doctor and surgeon at Notting Hill, London. Menzies was responsible for the discovery of numerous species and introducing many new plants to Europe, none acquired more unusually than the monkey puzzle tree, Araucaria araucana (Molina) C. Koch. While visiting the capital of what is now Chile, Vancouver and Menzies were entertained by the Spanish Viceroy Don Ambrosio O'Higgins, of Irish birth and whose illegitimate son would later lead Chile to independence. Menzies was unable to identify the large seeds that his host had provided as a delicacy, so he put some in his pocket to identify later. On the return journey, several sprouted and were introduced to English gardens where they caused something of a sensation. A great many plants associated with the travels of Menzies around the world are named in his honour, including the genus Menziesia Sm. in the Ericaceae. Menzies received honorary degrees from the Universities of Aberdeen and Leipzig and was elected 'father of the Linnean Society' in 1842, just one month before he died.

Menzies forwarded original material to Joseph Banks from the expedition with Captain Vancouver, the specimens later being bequeathed to Robert Brown (1820) and subsequently transferred to BM in 1827. Though the main herbarium of Menzies is generally listed at E, another collection which included the herbarium of John Zier was acquired by BM in exchange from New College, Edinburgh (1886). Menzies sent cryptogamic material, particularly bryophytes, to W.J. Hooker who published the novelties but the holotype material which was deposited at K was subsequently transferred to BM (c. 1961) under the terms of the Morton Agreement.