This paper proposes the Mobile MH-MAC (Multimode Hybrid - Medium Access Control) protocol, an extension of MH-MAC with enhanced support for mobility on wireless sensor networks. It provides an asynchronous mode for energy efficiency during idle (unconnected) periods, and a synchronous mode for high throughput periods. Mode change is controlled by the application in a cross-layer approach. MMH-MAC was specially designed to tolerate asyn- chronous mobile nodes in the neighborhood of synchronous nodes, providing fast transition and interference mitigation mechanisms. Current MAC protocols rely on a passive method to associate a mobile node to a static cluster, triggered by the reception of a beacon. We propose an active approach that allows a mobile node to start sending data packets shortly after entering into data range of a static node, with negligible data loss in existing synchronous data connections. MMH-MAC was implemented in TinyOS and tested on TelosB motes and TOSSIM simulator.