Winn takes on the top role next month following Rebecca Hill's promotion to become V-P of global corporate comms, based in New York.
He leaves ABN Amro after nearly four years, handing the role of global head of media relations for the wholesale clients unit to Steven Blaney.
Blaney was previously head of media and public affairs for ABN Amro Australia and New Zealand, based in Sydney.
At S&P, Winn will be in charge of heading all corporate comms for the group in Europe - an expansion of the role, which formerly oversaw PR for just the credit ratings side of the business.
He reports to Hill and S&P European managing director Francois Veverka.
He will head a core team of four in London, as well as supervising comms staff in the company's European offices in Stockholm, Moscow, Frankfurt, Milan, Madrid and Paris.