He was born in 1659, the third son of Sir John Ponsonby (c 1609 – 1678) of Bessborough in county Kilkenny, a colonel of Horse in the Commonwealth service, and the second son by Sir John"s second wife Elizabeth, widow of Edward Trevor and daughter of Henry Folliott, 1st Baron Folliott.
Trinity College.
Bessborough, formerly known as Kildaton, had been confiscated from Edmund Daton, whose family had held it since the 13th century, for his part in the Irish Rebellion of 1641. William Ponsonby matriculated as a fellow-commoner at Trinity College, Dublin, on 14 November 1677, at the age of eighteen. On 11 September 1721 he was created Baron Bessborough, of Bessborough, county Kilkenny, in the peerage of Ireland, and took his seat in the Irish House of Lords on 23 September.
He was further created Viscount Duncannon, of the fort of Duncannon, county Wexford, on 28 February 1723.
Ponsonby married Mary, daughter of the Honorary Randle Moore and granddaughter of Charles Moore, 2nd Viscount Drogheda.
Ponsonby sat in the Irish House of Commons for county Kilkenny from 1692 to 1693, from 1695 to 1699 and from 1703 to 1721. He was appointed a member of the Irish Privy Council on 11 November 1715.
