Myelin improves axonal conduction velocity and is essential for nerve development and regeneration. In peripheral nerves, Schwann cells depend on bidirectional mechanical and biochemical signaling to form the myelin sheath but the mechanism underlying this process is not understood. Rho GTPases are integrators of "outside-in" signaling that link cytoskeletal dynamics with cellular architecture to regulate morphology and adhesion. Using Schwann cell-specific gene inactivation in the mouse, we discovered that RhoA promotes the initiation of myelination, and is required to both drive and terminate myelin growth at different stages of peripheral myelination, suggesting developmentally-specific modes of action. In Schwann cells, RhoA targets actin filament turnover, via Cofilin 1, actomyosin contractility and cortical actin-membrane attachments. This mechanism couples actin cortex mechanics with the molecular organization of the cell boundary to target specific signaling networks that regulate axon-Schwann cell interaction/adhesion and myelin growth. This work shows that RhoA is a key component of a biomechanical response required to control Schwann cell state transitions for proper myelination of peripheral nerves.