John inherited the family estate at Tullycarn, Ireland by 1727, at about age eleven. A sea merchant in the American and West Indies. He was probably in opposition to the King's repressive, anti-colonial, anti-Irish, anti-Presbyterian mercantile policies. Coupled with the ruinous state of Ireland's economy, these factors may have caused him to have a home in more than one port. There is little mention of his activities in the shipping records of the colonies or England. A search of Lloyd's Daily Lists over a twenty year period reveals only two "M'gill" with voyage arrivals, one at Cork in 1757 and one at what was possibly his home port of Newry in 1753, both originating in the Caribbean. Perhaps this absence of regular citations was the result of most of his voyages not clearing customs nor carrying hull insurance. Smugglers left few records. He apparently traded in rum and exports from the North American colonies.
Around 1765 John moved his base of operations to Middletown, Connecticut, a port similar to Newry with its upland situation, well-concealed form the open sea. There is, however, no evidence that he ever established permanent residence there. He and his first mate, nephew James Magill, were loyalists. By 1760 Middletown was the largest and most prosperous town in Connecticut., with a population exceeding five thousand. Seventeen of the families living on Main Street were engaged in the sea trade, principally focused on the West Indies, especially rum. Although loyal to the crown, it is said that in 1775 his ship put in at Boston just before the Battle of Bunker Hill . He tried in vain for some days to persuade the dockhands to help him unload his ship, which they refused to do unless his crew declared support for the colonial patriots. Captain John's reply to this was " Let the damned Yankees stew in their own juice!" Captin John was not heard from again in New England. One unsubstantiated family source asserts that he left his first wife well-provided for in Ireland, and married a second, Mary Clough, in Middletown. He apparently also left her, and may have been last heard of in Glasgow.
(Ziebarth, John. Direct conections : Ziebarth-McGill ancestry. New York : Ziemag Publishing, 1997, pg. 82).