Matt Peacock, currently group director of comms with global energy firm BG Group, will join in June.
Leach, who has worked in media and comms roles at Vodafone for a decade, will stay at the mobile telecoms firm until June, to hand over to Peacock.
Leach told PRWeek he had made 'no decisions at this point' about his plans after leaving the firm.
Peacock joined BG Group in 2008, having previously headed comms for UK comms regulator Ofcom. He has also served as director of external relations at mobile telecoms group 3 and as UK chief comms officer at AOL, as well as working as a BBC broadcast journalist.
The news came as Vodafone sold its stake in France's second biggest mobile-phone operator, SFR, to Vivendi for £6.8bn this week.
Vodafone's retained financial agency Finsbury handled the sale of its 44 per cent stake, with Capital MS&L supporting the comms of Vivendi.
The Capital MS&L team is being headed by MD Steffan Williams. The agency previously advised Vivendi on its acquisition of Brazilian telecoms company GVT in 2010.