A water inflow impervious manhole cover assembly includes a molded non-metallic frame, an imperforate manhole cover and a fastening mechanism releasably interconnecting the frame and the cover. The frame includes an annular peripheral frame wall defining a cover-receiving seat with a radially inwardly extending lip having a seating surface with an annular sealing channel formed therein. The fastening mechanism includes cams rotatable relative to the frame between a locked position wherein the cams engage the frame and an unlocked position wherein the cams are disengaged from the frame for removal of the cover. An annular watertight seal is formed between a bottom planar surface of the cover and the seating surface of the frame and a gasket retained within the annular sealing channel, when the cams of the fastening mechanism are disposed in the locked position.