Patterned Mg2Si structures can be attractive for use in optoelectronic, thermoelectric, and other devices. Such structures can be fabricated as conformal Mg2Si films on patterned Si substrates via direct reaction of these substrates with Mg vapor at modest temperatures and ambient pressure. Here, we report the kinetic mechanism (rate-limiting step and associated rate law) that controls such vapor/solid reactive formation of conformal Mg2Si films. To avoid oxidation during silicide formation, experiments were conducted within metal ampoules that were sealed in an argon atmosphere. Upon heating each sealed ampoule to 600 degrees C, Mg vapor (generated from solid Mg sealed within the ampoule) underwent reaction with a Si single-crystal plate (located a fixed distance from the solid Mg within the ampoule). Kinetic data were obtained by evaluating the change in thickness of the product Mg2Si film, and the specimen mass change per area, with reaction time at 600 degrees C. The changes in mass and in film thickness were both found to follow parabolic kinetics, with the rate constants obtained from both types of measurements in good agreement. Changes in the diffusion distance of the Mg vapor to the reacting Si single-crystal plate, and in the crystal orientation of the plate, were found to have negligible influences on the reaction kinetics. Such data were consistent with solid-state diffusion through the Mg2Si product layer as the rate-limiting step. An inert marker experiment was used to determine the predominant interface at which Mg2Si formed, and to evaluate the rate-limiting species controlling such formation.