Mellish, who joined the asset manager’s UK arm in January 2010, becomes head of UK/Ireland business, MENA and global consultant relationships and he will be based in London.
During his career at NGAM he was variously head of UK/Ireland institutional business, and business development unit head for the MENA region.
He is also a member of the NGAM international executive committee, which makes him part of the senior management team responsible for business outside of the US.
Mellish previously worked for Union Bancaire Privée, Credit Suisse Asset Management, and Schroder Investment Management.
A company spokeman confirmed that Jamal Saab had left the company.
Earlier this year NGAM appointed Moad Touhami as director and head of distribution for the Dubai office.