A speed control mechanism for controlling selective engagement of the self-propelled transmission assembly for a walk-behind lawn mower is provided. The speed control mechanism is also configured to control the relative output speed of the transmission assembly. The speed control mechanism includes a speed engagement assembly and a speed adjustment assembly, wherein the speed engagement assembly includes a pair of rotatable levers for causing the transmission assembly to actuate between a disengaged position and an engaged position and the speed adjustment assembly includes a rotatable knob for causing the relative output speed of the transmission assembly to the wheels of the lawn mower to increase or decrease.