He will become head of corporate comms, Europe and Africa, when he joins Cargill in November, reporting to European comms director Geraldine O'Shea.
Cargill, the world's largest agribusiness firm, has food production facilities all over the world from soya production outfits in Brazil to UK chicken supplier Sun Valley. It also has a financial and risk-management business, and reported profits of £919m in the 12 months to March this year.
MacGregor's resignation has prompted a review of the comms director role at RCCL, which is yet to announce whether he will be replaced directly or the role redefined.
MacGregor only joined RCCL - which uses Siren PR - last summer after being made redundant from his EMEA comms manager post at Coca-Cola (PRWeek, 22 July 2005).