He was born in Limerick, into one of the city's most politically influential families, elder son of the Rev.

A trained barrister,[2] Pery became a member of the Irish House of Commons for the Wicklow constituency in 1751.

He was considered one of the most powerful politicians in Ireland in his time, leading a faction which included his nephew the future Earl of Limerick and his relatives by marriage, the Hartstonges.

William's son, Edmund, was made Earl of Limerick in 1803 as a result of his support for the Act of Union.

Pery's younger sister was Lucy Hartstonge, the founder of what is now St John's Hospital.