Fieldhouse, a former BSMG Worldwide deputy CEO, replaces Jacqui Reid (née Crane), who will leave before the end of the year.
Hargreaves, who has worked at WS for four years, joins Edelman next month. Prior to WS, he spent five years at Burson-Marsteller.
Reid was head of corporate PR at Edelman client Microsoft before joining the agency two years ago (PRWeek, 4 September 2003).
At Microsoft she implemented its first group-wide corporate PR strategy.
Edelman London CEO Stuart Smith said the appointments 'signal our continued commitment to invest in our corporate and technology offerings'.