The charter of incorporation for St Johnstown is missing, but from other charters it was known to have been incorporated before 1619. It was the object of a corporation takeover. The original patrons and lords of the soil were the Earls of Abercorn. In 1736 William Forward (0802) of Castle Forward was elected Provost of St Johnstown, and he contrived to gain control of the corporation. In 1754 the Earl of Abercorn was seeking legal advice about the corporation. However, Forward retained his control and bequeathed it to his daughter and heiress, Alice Howard, née Forward, later Countess of Wicklow.
The 1st Marquess of Abercorn resented the loss of what had been an Abercorn borough and was most anxious for its return:in February 1790 he wrote to Henry Pomeroy (1694) that 'Between ourselves, I am trying to sound Lord Wicklow whether he will sell me St Johnstown. I cannot bear not to recover that family borough if possible.' Lord Abercorn even prevailed on Chief Secretary Hobart to discover whether a borough in Co. Wexford, which he had heard was for sale, would be acceptable to Lord Wicklow.133 All to no avail: the Countess of Wicklow was equally anxious to retain St Johnstown and she was awarded the £15,000 compensation at the Union. Lord Abercorn had to be satisfied with the Co. Tyrone borough of Augher.