He was born in 1659, the third son of Sir John Ponsonby (c. 1608/9 – 1678) of Bessborough in County Kilkenny, a Cromwellian colonel of Horse in the Commonwealth service, and the third son by Sir John's second wife Elizabeth, widow of Richard Wingfield and of Edward Trevor, and daughter of Henry Folliott, 1st Baron Folliott.
Folliott Wingfield, 1st Viscount Powerscourt, was his older half-brother on his mother's side.
Bessborough, formerly known as Kildalton, had been confiscated from Edmund Dalton, whose family had held it since the 13th century, for his part in the Irish Rebellion of 1641.
It was granted to Sir John Ponsonby who renamed it after his second wife Elizabeth (Bess).
William Ponsonby matriculated as a fellow-commoner at Trinity College Dublin, on 14 November 1677, at the age of eighteen.