Herein we demonstrate that an external electric field (EEF) acts as an accessory catalyst/inhibitor for Diels-Alder (DA) reactions. When the EEF is oriented along the "reaction axis" (the coordinate of approach of the reactants in the reaction path), the barrier of the DA reactions is lowered by a significant amount, equivalent to rate enhancements by 4-6 orders of magnitude. Simply flipping the EEF direction has the opposite effect, and the EEF acts as an inhibitor. Additionally, an EEF oriented perpendicular to the "reaction axis" in the direction of the individual molecule dipoles can change the endo/exo selectivity, favouring one or the other depending on the positive/negative directions of the EEF vis-à-vis the individual molecular dipole. At some critical value of the EEF along the "reaction axis", there is a crossover to a stepwise mechanism that involves a zwitterionic intermediate. The valence bond diagram model is used to comprehend these trends and to derive a selection rule for EEF effects on chemical reactions: an EEF aligned in the direction of the electron flow between the reactants will lower the reaction barrier. It is shown that the exo/endo control by the EEF is not associated with changes in secondary orbital interactions.