A mobile wireless communications device may include a housing carrying a circuit board and wireless communications circuitry. An antenna assembly is carried by the housing and coupled to the wireless communications circuitry. The antenna assembly may include an electrically conductive base having a rectangular shape with opposing first and second ends and opposing first and second sides extending between the first and second ends. The electrically conductive base may have an elongate slot therein extending within a medial portion thereof and contained within the opposing first and second ends and the opposing first and second sides. The antenna assembly also may include an electrically conductive feed arm extending outwardly from the first side of the electrically conductive base adjacent the first end thereof. The electrically conductive feed arm may have a distal end with an antenna feed defined thereon.