He was born at Newtown, County Offaly, the son of William Smith (died 1747) and his wife Hester Lynch of Galway.

He was elected member of the Irish House of Commons for Randalstown in 1783, and was noted for his reason and moderation in debate, despite a rather "stiff and monotonous" delivery.

The Mastership of the Rolls had long been a notorious sinecure for politicians, many of whom had no legal qualifications whatever, and some of whom were Englishmen who rarely visited Ireland.

William's second son Thomas Berry Cusack Smith continued the family traditions of judicial eminence and oddity: like his grandfather, he was Master of the Rolls in Ireland, and like his father, he was notably eccentric.

[2] According to Elrington Ball, he was noted for learning and eloquence; in contrast to his son and grandson, who were both notably hot-tempered, he was invariably calm and self-controlled.

Daniel O'Connell, then a rising young barrister, who thought poorly of Irish judges in general, complained of Smith's inefficiency, yet praised him as "a gentleman and a scholar, polite, patient and attentive".