He was a council member for a decade and was a garden advisor for RHS Hyde Hall. MacLeod also chaired the RHS science and horticultural advice committee.
He was director of the National Institute of Agricultural Botany, chairman of the British Beet Research Organisation, vice-president of the International Institute for Research on Beet and a board member and director of the National Non-Food Crops Centre.
RHS president Giles Coode-Adams said: "John had been a tremendous support to me during my presidency and we all appreciated his wise counsel. We are really saddened by the death of this outstanding man and our sympathies go out to his family."