A system and method control a trim device that positions a trimmable marine apparatus with respect to a marine vessel. A trim system is operated in an automatic mode, in which a controller sends signals to actuate the trim device automatically as a function of vessel or engine speed, or a manual mode, in which the controller sends signals to actuate the trim device in response to commands from an operator input device. An operating speed of the propulsion system is determined. When the operating speed has crossed a given operating speed threshold, the trim system is subsequently operated in the automatic or manual mode depending on whether the operating speed increased or decreased as it crossed the operating speed threshold and whether the trim system was operating in the automatic or manual mode as the operating speed crossed the operating speed threshold.