Muncaster, a former sales director at Eurosport and CNN, is leaving Metro to run his own business in Scotland.
Metro has promoted Greg Miall, the head of pan-European advertising, to the role of head of international advertising to compensate for the loss of Muncaster.
Muncaster joined Metro, which has 38 editions in 16 countries, in 2002 from CNN. He is relocating to Scotland to be closer to his family.
Miall, 30, joined Metro as its international account director two years ago and helped Muncaster to build its international sales team. He then became the managing director of Metro in Marseille and then the commercial director of Metro Poland before returning to London last year.
Muncaster has spearheaded Metro's efforts to increase sales from multimarket deals with advertisers including BA, Dell and Buena Vista. Miall will attempt to build on this activity.