 William Penn became a member of the Institution of Mechanical Engineers in 1873, proposed by his father and Joseph Whitworth.He became a partner in the family business around the same time, began to manage the firm in 1875, and in 1889, when the firm was incorporated as John Penn and Sons Ltd, he and his elder brother John were the two principal shareholders.When the company amalgamated with the Thames Ironworks and Shipbuilding Company ten years later, William Penn became a director of the new company, but resigned from the board by April 1901 on health grounds.