This paper extends the formalism of prioritized synchronous composition (PSC), proposed by Heymann for modeling interaction (and control) of discrete event systems, to permit system interaction with their environment via interface masks. This leads to the notion of masked prioritized synchronous composition (MPSC), which we formally define. MPSC can be used to model interaction of systems at single as well as multiple interfaces. We show that MPSC can alternatively be computed by "unmasking" the PSC of "masked" systems, thereby establishing a link between MPSC and PSC, We next prove that MPSC is associative and thus suitable for modeling and analysis of supervisory control of discrete event systems. Finally, we use MPSC of a discrete event plant and a supervisor for controlling the plant behavior and show (constructively) that under the absence of "driven" events, controllability together with normality of the given specification serve as conditions for the existence of a supervisor. This extends the results on supervisory control, which permits control and observation masks to be associated with the plant only.