William Mowbray was born in Leicester, Leicestershire, England, on 25 December 1835, the son of William Mowbray, a lace manufacturer, and his wife, Anne. He received some of his education under Canon William Fry, and trained as a teacher for three years at St Mark's College, Chelsea, run by the National Society for Promoting the Education of the Poor in the Principles of the Established Church. From 1856 to 1858 Mowbray taught in Leicester, where he married Emma Bird on 24 February 1857; they were to have four daughters and three sons.
In 1858 William Mowbray was invited by the Church of England Education Society in Wellington, New Zealand, to take charge of St Paul's School in Sydney Street at a salary of £150 per annum. One of the society's two primary schools in the town, St Paul's had been opened on 5 April 1852 with 35 pupils. The following year there were 105 on the roll, but by the end of the decade, a too-frequent change of masters left the school 'sadly fallen off'; there were, for example, only enough slates for one class at a time. Mowbray, his wife and infant son arrived in Wellington on the Midlothian on 5 February 1859.
Mowbray's enthusiasm, his insistence on high standards, and his wide-ranging interests effected a gradual improvement in the school's fortunes, and it came to be known informally as Mowbray's school. The roll included older boys, some of them Maori, who learned Latin and geometry and other subjects normally associated with secondary education. Music and drawing were taught, and Mowbray fitted up a science room where he gave lessons in electricity and chemistry. Punishment was delivered, when necessary, by the 'Mowbray arm and the Mowbray supplejack'.
The broad curriculum at St Paul's influenced other schools, and the Wellington Teachers' Association, which later became the Wellington branch of the New Zealand Educational Institute, elected Mowbray its first chairman in 1874. Throughout his professional life he kept up to date with new developments in education. For a period in the 1880s he trained pupil-teachers in English and physics for the Wellington Education Board. When St Paul's was taken over by the provincial education board in 1873, he was retained as its headmaster, continuing in that role when the school was renamed Thorndon School and then moved to a new site in 1880. He retired in 1902, by which time he had held the post for 43 years.
Thorndon School's prosperity and harmony, it was said, spoke eloquently of Mowbray's powers of control. By the end of his career a large number of people in prominent and responsible positions, including his successor, J. C. Webb, had been taught by him, and New Zealand had profited from the 'particular degree of excellence' he encouraged.
Mowbray's other legacy was in the field of music. Both he and his wife from the time of their arrival in Wellington attended St Paul's Church (later St Paul's Cathedral Church) where William was a member of the vestry in the 1860s. Emma played the harmonium for nine years until the arrival of the first pipe organ, and William also acted as organist at times. He conducted the choir for over 19 years and kept it together 'without any bickerings'. When the vestry decided to combine the positions of organist and choirmaster, Mowbray retired in Robert Parker's favour in 1878 but continued to sing in the choir under Parker's direction. With A. J. Johnston, Mowbray founded, in October 1860, the first Wellington Choral Society, which he conducted for 14 years.
In 1875 Mowbray bought from Robert Park a section adjacent to the cemetery in Bolton Street and subdivided it to include the street which was later named after him. His property in Wellington was valued at £2,800 in the 1880s, and he also owned 46 acres in Manawatu. He was an original member of the Thorndon Bowling Club, and belonged to the Hutt Bowling Club up to the time of his death. William and Emma Mowbray lived for about 20 years in Sydney Street, then moved to Lower Hutt, where Emma died on 22 December 1912, aged 76; William died on 21 June 1916 in his 81st year. Their grave is in the Taita cemetery.
The alert and bearded Mowbray, by his innovations and his personality, contributed much to the general level of education in mid-Victorian Wellington. He was known as a man of unvarying courtesy and kindly sympathy, a tactful administrator, a faithful friend, a generous and sympathetic critic and an impartial judge. He has a citation in the Book of Commemoration in Old St Paul's.