How physical force is sensed by cells and transduced into cellular signaling pathways is poorly understood. Previously, we showed that tyrosine phosphorylation of p130Cas (Cas) in a cytoskeletal complex is involved in force-dependent activation of the small GTPase Rap1. Here, we mechanically extended bacterially expressed Cas substrate domain protein (CasSD) in vitro and found a remarkable enhancement of phosphorylation by Src family kinases with no apparent change in kinase activity. Using an antibody that recognized extended CasSD in vitro, we observed Cas extension in intact cells in the peripheral regions of spreading cells, where higher traction forces are expected and where phosphorylated Cas was detected, suggesting that the in vitro extension and phosphorylation of CasSD are relevant to physiological force transduction. Thus, we propose that Cas acts as a primary force sensor, transducing force into mechanical extension and thereby priming phosphorylation and activation of downstream signaling.