William Thomas was born on 8 December 1879 in Dunedin, the eldest child of Jane Little and her husband, Nicholas Thomas. Nicholas had been a goldminer in Australia before coming to New Zealand in 1863. Here he prospected for gold in Central Otago and made frequent visits to Dunedin. His itinerant lifestyle made for long absences from home. Anxious to avoid separation, Jane took William, barely six months old, on horseback to join her husband at Mt Buster. For the next six months they lived in a sod cottage, before returning to Dunedin where two more children were born.
Jane sent William to her old primary school, Union Street. There he made rapid progress scholastically in addition to demonstrating a talent and an enthusiasm for sports and games. When the family moved from Dunedin to Waimate at the end of 1892, William had passed the standard five examination with distinction. He continued to thrive, becoming dux of Waimate School for 1893. He also displayed considerable ability in both cricket and rugby, representing South Canterbury in both sports. He studied voice training and flute playing, and took an active part in the Waimate Presbyterian Sunday school. In 1894 William enrolled in a full programme of secondary school studies at Waimate District High School and passed the first examination for public school teachers in 1894.
Thomas's admiration for, and friendship with, John Smyth, his headmaster, influenced his decision to embark on a teaching career. With Smyth's backing he was accepted as a pupil-teacher at Waimate District High School beginning in 1895 at a salary of £27 per year. For the next four years he taught classes ranging from infants to form six. In his fourth and final year he passed both the final Teachers' Class D and the University of New Zealand's matriculation examinations.
Promoted to assistant teacher at the same school in 1899, Thomas became head of Waitaki School (1900–1903), Morven (1903–5), Pleasant Point District High (1906–10), and Waimataitai schools. He married Ethel Margaret Jones – they had met earlier at the Waimate Presbyterian Sunday school – on 4 July 1905, at Waimate. At Pleasant Point Thomas completed his BA and MA (in 1906 and 1908 respectively); in 1921 he completed his LLB degree.
With ill health forcing the retirement of the Timaru Boys' High School rector in 1912, the board advertised for a replacement. Thomas, the only primary school headmaster to apply, was appointed from a very strong field of 35 applicants. He assumed his duties on 1 January 1913. Within months he had broadened the school curriculum to include agriculture, art, drama, music, woodwork and wool-classing. Additional staff were appointed, new buildings were erected, a preparatory school was established, and the boarding school was expanded.
Remembered by many of his colleagues and pupils as 'Old Bill', 'The Boss' or 'Walrus Bill', he was thickly set, balding, and sported a large black moustache. He was an inspiring headmaster and teacher with a deep love of English literature, a successful sports coach and player, and a brilliant public speaker. Throughout his 22 years as rector, Thomas gave generously to school projects. He personally paid for band instruments, for the tennis courts to be resurfaced, and for a section of land for the school. He donated (anonymously) £1 per week for each married relief worker employed on school property during the depression, and provided Christmas and wedding presents to all his staff.
Thomas's retirement years were no less busy than his working ones. Retiring in March 1935, he continued his long association with Rotary: he was first president of the Timaru Rotary Club, for a time governor for the area which encompassed New Zealand and Fiji, and presided over the Sixth Pan-Pacific Conference of Rotary International, held in Wellington in March 1937. He also became president of the South Canterbury branch of the New Zealand Crippled Children Society, was a specially appointed justice of the peace working in the Children's Court, and the New Zealand representative for Longmans, Green and Company, an English educational book publisher. Thomas ventured briefly into politics when he was persuaded to stand for Parliament as an independent Nationalist in the 1935 general election. As one who had encountered few setbacks in his career, Thomas was deeply hurt when defeated by Labour's Clyde Carr.
Soon after, he was invited by the New Zealand Council for Educational Research to join its director, Dr C. E. Beeby, and M. H. Oram, a statistician, to investigate university entrance. Their book, Entrance to the university, was published in 1939. Three years later H. G. R. Mason, the minister of education, appointed Thomas chairman of a consultative committee on the post-primary school curriculum. Their report, the Thomas Report, recommended substantial reforms in secondary education by the introduction of a common core curriculum for all types of secondary education institutions: all children were to receive a sound general education. The report's recommendations helped to shape secondary education in New Zealand for 40 years.
Thomas was unable to join in the debate that followed the release of the report. He died as a result of heart failure while attending a friend's daughter's wedding at Timaru on 25 August 1945. He was survived by his wife; the couple had no children.