Earl of Londonderry extinct on the death of his great-grandson, the fourth Earl, in 1714. The second creation came in 1726 in favour of the soldier and politician Thomas Pitt, 1st Baron Londonderry. He had already been created Baron Londonderry in 1719 and was made Viscount Gallen-Ridgeway at the same time as he was granted the earldom, also in the Peerage of Ireland. He was the husband of Lady Frances Ridgeway, daughter and heiress of the fourth Earl of the first creation. A member of the prominent Pitt family, he was the uncle of William Pitt, 1st Earl of Chatham. His younger son,
Earl of Seaforth would have been the 6th earl, helped the British government during the rising of 1745, and was a member of parliament for some years. His son Kenneth Mackenzie was created Baron of Ardelve and Viscount Fortrose in the Peerage of Ireland in 1766 and Earl of Seaforth in 1771, also in the Peerage of Ireland. However, these peerages became extinct when he died in August 1781. Although there were still heirs to the older earldom, this remained under attainder. Kenneth raised the regiment of Highlanders, the 78th (later known as 72nd) in 1778, known later as the 1st battalion of