Henderson, who has also been with the firm for almost three decades, has recently been based in Australia to oversee the business William Hill bought from Sportingbet last year. William Hill is yet to officially announce the appointment but Topping has spoken in the past about his eagerness to be succeeded by an internal candidate.
In an interview with The Telegraph newspaper last year, he said he was keen for the company to keep hold of talent, referring to Henderson, as well as finance director Neil Cooper.
Topping also said in February of this year that he would not play a part in vetting the final candidates, despite being asked to do so by chairman Gareth Davis.