The 47-year-old takes over from Duff who had been at the club for 23-years. Cullen admits football will provide a fresh challenge after a 25-year career in the hospitality trade.
He said: "I have always followed football so I know it's different from most businesses and there is a profile and expectation associated with the post, from both supporters and customers.
"I'm determined to hit the ground running and get myself out and about locally."
Saints vice-chairman Steve Brown added: "We had more than 70 applicants for the position and there were many excellent candidates.
"In all, the process took six weeks but, in appointing Grant, we believe we have the right man for St Johnstone."
Source: Team Talk
Source: Team Talk