In 1965 Stouthuysen was officially appointed director of the company. In 1971 he was appointed as a member of the board of directors and in 1987 he became the director and in 1988 president of Janssen Operations Worldwide.
Stouthuysen's career progression involved significant promotions, starting with his appointment as head of the company in 1965. He then joined the board of directors and continued to rise through the ranks until he became director and ultimately president of Janssen Operations Worldwide in 1988.