Simpson takes charge following Sir Elton John's decision to step down as club chairman last May.
Simpson was thrilled by his appointment, saying: "I'm very proud and honoured. It surpasses any dream I had as a boy standing on the terraces.
"However my pride and joy are tempered by the financial situation we are in."
With the club in such financial difficulties that it was forced to ask the players to agree to deferred payment of 12.5 per cent of their salaries this week, the new Chairman faces a tough task to keep the Hornets in business
"I'm under no illusions and I am absolutely committed to turning things round. The last few days have been difficult but we have pulled no punches," he said.