While at the bookshop, he was offered a position as a librarian at the Royal Horticultural Society in London (1933–1952). From there he moved to the Natural History Museum as a scientific officer in the botany department (1952–1976). After his retirement, he continued working there, writing, and serving on a number of professional bodies related to his work, including the Linnean Society, of which he became president. He also taught botany at Cambridge University as a visiting professor (1977–1983).
Stearn is known for his work in botanical taxonomy and botanical history, particularly classical botanical literature, botanical illustration and for his studies of the Swedish scientist Carl Linnaeus. His best known books are his Dictionary of Plant Names for Gardeners, a popular guide to the scientific names of plants, and his Botanical Latin for scientists.
Stearn received many honours for his work, at home and abroad, and was made a Commander of the Order of the British Empire (CBE) in 1997. Considered one of the most eminent British botanists of his time, he is remembered by an essay prize in his name from the Society for the History of Natural History, and a named cultivar of Epimedium, one of many genera he produced monographs on. He is the botanical authority for over 400 plants that he named and described.
William Thomas Stearn was born at 37 Springfield Road, Chesterton, Cambridge, England, on 16 April 1911, the eldest of four sons, to Thomas Stearn (1871 or 1872–1922) and Ellen ("Nellie") Kiddy (1886–1986) of West Suffolk. His father worked as a coachman to a Cambridge doctor. Chesterton was then a village on the north bank of the River Cam, about two miles north of Cambridge's city centre, where Springfield Road ran parallel to Milton Road to the west. William Stearn's early education was at the nearby Milton Road Junior Council School (see image). Despite not having any family background in science (though he recalled that his grandfather was the university rat-catcher) he developed a keen interest in natural history and books at an early age. He spent his school holidays on his uncle's Suffolk farm, tending cows grazing by the roadside where he would observe the wild flowers of the hedgerows and fields. Stearn's father died suddenly in 1922 when Stearn was only eleven, leaving his working-class family in financial difficulties as his widow (Stearn's mother) had no pension.
That year, William Stearn succeeded in obtaining a scholarship to the local Cambridge High School for Boys on Hills Road, close to the Cambridge Botanic Garden, which he attended for eight years till he was 18. The school had an excellent reputation for biology education, and while he was there, he was encouraged by Mr Eastwood, a biology teacher who recognised his talents. The school also provided him with a thorough education in both Latin and Greek. He became secretary of the school's Natural History Society, won an essay prize from the Royal Society for the Protection of Birds and spent much of his time at the Botanic Garden. Stearn also gained horticultural experience by working as a gardener's boy during his school holidays, to supplement the family income.
Stearn attended evening lectures on paleobotany given by Albert Seward (chair of botany at Cambridge University 1906–1936), and Harry Godwin. Seward was impressed by the young Stearn, giving him access to the herbarium of the Botany School (now Department of Plant Sciences—see 1904 photograph) and allowing him to work there as a part-time research assistant. Later, Seward also gave Stearn access to the Cambridge University Library to pursue his research.
Stearn was largely self-educated and his widowed mother worked hard to support him while at school but could not afford a university education for him, there being no grants available then. When not at the Botany School, he attended evening classes to develop linguistic and bibliographic skills. His classes there included German and the classics. He obtained his first employment at the age of 18 in 1929, a time of high unemployment, to support himself and his family. He worked as an apprentice antiquarian bookseller and cataloguer in the second-hand section at Bowes & Bowes bookshop, 1 Trinity Street (now Cambridge University Press), between 1929 and 1933 where he was able to pursue his passion for bibliography. During his employment there, he spent much of his lunchtimes, evenings and weekends, at the Botany School and Botanic Garden. This was at a time when botany was thriving at Cambridge under the leadership of Seward and Humphrey Gilbert-Carter.