A hydraulic mount assembly is disclosed having a partition including a hydraulic damping decoupler between two hydraulic chambers. One chamber is formed by an elastomeric member and the other by a resilient diaphragm. During dynamic loading of the mount, fluid passes between the two chambers of the mount causing expansion and contraction of the diaphragm. A pneumatic bladder is positioned to exert external pressure against the diaphragm in order to change the damping characteristics of the mount. As the bladder is expanded, the diaphragm is stiffened and the fluid flow between the two chambers in the hydraulic mount is restricted. When the bladder is deflated, the pressure against the diaphragm is reduced or effectively removed thereby restoring full freedom of movement of the fluid in the hydraulic mount. By actively controlling the diaphragm expansion in this manner, the dynamic characteristics of the mount are varied. A control circuit with on-board transducers is provided to monitor vehicle operating and road response conditions and modulate the pressure to the bladder for maximum damping effect. The on-board transducers sense selected parameters to indicate unusual conditions for which modulation is required, such as rough engine operation, engine lugging, rough road conditions, sudden turning and/or rapid acceleration/deceleration.