John Campbell (1762-1834) succeeded to the title upon the death of his cousin, the 3rd Earl in 1782. He married Mary Turner, co-heir of David Gavin of Langton, and had issue. He raised in 1793, two regiments of Fencible Infantry and later a third known as the Breadalbane Fencibles of which the Earl was Colonel. A coattee that is thought to have been the Earl's is now in the Scottish United Services Museum.

A portrait of the 1st Marquess by Sir Henry Raeburn is in the Scottish National Portrait Gallery (PG2358).