A barber by trade, apparently from the north-east, he was accepted on trial at the 1749 Conference and stationed in London. Wesley not only stationed him in Ireland, but made him General Superintendent over al the preachers in Ireland from at least July 1750 to Easter 1751. In September Charles Wesley carried out a major purge of the northern preachers and Fenwick was one of the ten preachers dropped. Charles described his preaching as pure, unmixed nonsense and made him set up again in his own trade as a barber. By 1754 John Wesley had allowed him to return as an itinerant in the Manchester Round, partly because he had found him useful as a travelling companion in both England and Ireland.
In 1757 he gained a coveted mention in Wesleys Journal by falling asleep under a haystack during Wesleys preaching at Clayworth, near Retford. He then caused trouble by spreading rumours about Thomas Walsh being about to marry a widow and was dropped from the 1758 Minutes, only to be reinstated the following year. He appeared on the Stations between 1771 and 1779, but only under his initials, a sign that Wesley had reservations about him. Charles Atmore described him during the next few years as always attending the place of Conference but not permitted to be actually present at Conference after 1784.
We know from stray references that in January 1788 he was sent from Bolton to help Robert Dall in Dumfries and in 1789 was sent from Hexham to Bideford to help Samuel Bardsley mission the new area. He was bold enough to face the local Whig Lord Fortescue to demand protection from the mob. He continues to crop up as a preacher in many places, though according to Atmore he was not acknowledged as a [travelling] preacher for several years before his death but his preaching occasionally was connived at, and a small pittance of £12 was allowed him annually by the Conference to preserve him from want and distress.
He died after being struck by lightning in a mill where he was sheltering from a storm on the Wolds near Bridlington. His career shows how easy it was for Wesley to take the simple way out by letting the errant preacher back. He can be best described as the eternally repentant preacher.