When just fifteen years old, he volunteered as a militiaman, to perform duty in protecting the colonial frontier from incursions by the Indians.
An old chief, taking compassion on him, protected him from further injury, and on reaching the Lakes adopted Campbell, in whose family the young man remained during his three years' captivity.
While out on one of their hunting excursions, Campbell left the Indians, and after a fortnight's tramp through the pathless wilds reached the British.
The British commander was much interested in Campbell's account of his captivity and escape, and with his intelligence, and engaged him to pilot the army, which he did with success.
At the same time Campbell, along with Joseph Martin, began acting as an agent to the Indians, reporting back to Virginia governors Benjamin Harrison V, Edmund Randolph and others on the state of Indian-colonial relations.
In 1775 he was one of the 13 signers of the Fincastle Resolutions, the earliest statement of armed resistance to the British Crown in the American Colonies.