His mother was Elizabeth, daughter of Sir Thomas Harries, 1st Baronet, of Tong Castle, Shropshire.
Pierrepont was returned to Parliament as one of two representatives for Appleby in 1698, a seat he held until 1705.
[1] In 1702 he was raised to the Peerage of Ireland as Baron Pierrepont, of Ardglass.
This creation in the Peerage of Ireland allowed him to be ennobled, yet still remain in the House of Commons.
In 1714 he was further honoured when he was made Baron Pierrepont, of Hanslape in the County of Buckingham, in the Peerage of Great Britain, with this creation giving him an automatic seat in the House of Lords.