He spent two years at PBN as head of the company's financial communications department after joining from The Policy Partnership in 2006.
Leasor is one of the industry's leading names in the emerging market's space and has built with significant experience in Russia, the CIS and the Gulf since the early 1990s.
High profile clients he has represented include Telenor and Renaissance Group, and he has worked on the IPOs of M.video and Bank Saint Petersburg.
Leasor said: 'I have watched M: achieve truly spectacular results for many clients over recent years and it's good to be part of the team again. I've also been impressed by M:'s stellar growth over the last two years, in particular the benefits that being part of Sage Holdings has brought.'
He was at M: from 2003 to 2006 and has also worked as head of developing markets at FD.