A mobile wireless communications device may include a portable housing and wireless transceiver circuitry carried by the portable housing. The mobile wireless communications device may also include an antenna carried by the portable housing and coupled to the wireless transceiver circuitry, and a light source carried by the portable housing. The device may further include a keyboard assembly carried by the portable housing including a light guide coupled to the light source and having input key receiving openings therein, and a mask carried by the light guide and having input key receiving openings. The mask may include an electrically conductive layer positioned relative to the antenna to reduce a Specific Absorption Rate value for the mobile wireless communications device.