A mobile wireless communications device may include a housing having an opening therein and an inwardly extending housing ring adjacent the opening, and a switch carried by the housing and being aligned with the opening. The mobile wireless communications device may further include a backing member within the housing adjacent the opening, and an elastomeric switch membrane comprising a body covering the switch, and a flange coupled to the body and compressed between the backing member and the housing ring to define a seal therewith. The flange may include a flange base, and a pair of flange rings extending inwardly from the flange base and defining a valley therebetween aligned with the housing ring. In another embodiment, the flange may include a flange base and a first flange ring on a first surface thereof, and a pair of second flange rings on a second surface of the flange base and defining a valley therebetween aligned with the first flange ring.