In January 1970 the incumbent President of the NFU Gwilym Williams failed to get the 80% support necessary to be re-elected, and Plumb was elected President of the NFU in his place. Plumb's term of office included British accession to the European Economic Community and its Common Agricultural Policy and Plumb negotiated for greater support for British agriculture; he stressed that Britain no longer had the economic power to bring cheap food prices. Plumb was, however, a strong supporter of British membership of the European Economic Community; he was considered as a possible director for the pro-market campaign in the 1975 referendum campaign and was described as one of its most indefatigable spokesmen. Plumb was awarded a knighthood in the Queen's Birthday Honours list in 1973.