By the premature death of his father, the care of William and his elder brother Thomas devolved at an early age on their cousin, Mr. T.C.Brooksbank of the treasury under whom they were educated, and eventually placed in public offices.
He was deputy-registrar and registrar of the land-tax from 1819 to 1841, and was the author of valuable evidence on that subject given to the select committee on agricultural distress in 1836.
He was twice married: first, in 1827, to Ellen, daughter of Solomon Treasure, under-secretary for taxes, who died In 1819, by whom he had two sons, Frederick Brooksbank Garnett, created a C.B.
In 1851 he made a special visitation of all the assay offices in the United Kingdom, on which he reported to parliament, and valuable evidence on the subject was given by him to select committee of the House of Commons on 'gold and silver wares' in 1855 and 1856.
Garnett was not only distinguished for his long and eminent public services, but was in private life an admirable artist and musician.