The invention relates to a adjustable mounting assembly. There is a need for a simple adjustable and lockable mounting assembly. A mounting assembly adjustably supports a platform relative to a base. The assembly includes a plurality of adjustable length legs. Each leg has a first end pivotally coupled to the base and a second end pivotally coupled to the platform. An arm projects transversely away from each leg into the space surrounded by the legs. A locking or clamping device is located in the space surrounded by the legs and pivotally receives an inner end of each arm. The clamping device including a tension adjust member which can be tightened to prevent relative movement between the arms, and thus, the legs. The clamping device includes first and second plates which form a plurality of apertures therebetween. Each aperture pivotally receives a pivot ball on the inner end of each arm.