The arrival of the former FD managing director this week follows a series of key business wins for Powerscourt. In the past 12 months, the agency has advised BP through the Gulf of Mexico oil crisis and helped promote financial services company Prudential.
Sanderson joins as partner from management consulting venture NewMarket Partners.
Prior to joining NewMarket in 2010, Sanderson was managing director of FD's financial comms team. He left after 15 years, becoming one of the first big names to depart the firm after the completion of its earn-out period.
Sanderson will work alongside Powerscourt founder Rory Godson. He said: 'Giles is a huge figure in financial PR. It's a key appointment for us as we grow our highly differentiated business.'
Last month, PRWeek reported that Powerscourt had hired John Elliott, a former correspondent at the Sunday Times and commissioning editor at HarperCollins.