Integrin-mediated mechanosensing of the extracellular environment allows cells to control adhesion and signalling. Whether cells sense and respond to force immediately upon ligand-binding is unknown. Here, we report that during adhesion initiation, fibroblasts respond to mechanical load by strengthening integrin-mediated adhesion to fibronectin (FN) in a biphasic manner. In the first phase, which depends on talin and kindlin as well as on the actin nucleators Arp2/3 and mDia, FN-engaged alpha 5 beta 1 integrins activate focal adhesion kinase (FAK) and c-Src in less than 0.5 s to steeply strengthen alpha 5 beta 1- and alpha V-class integrin-mediated adhesion. When the mechanical load exceeds a certain threshold, fibroblasts decrease adhesion and initiate the second phase, which is characterized by less steep adhesion strengthening. This unique, biphasic cellular adhesion response is mediated by alpha 5 beta 1 integrins, which form catch bonds with FN and signal to FN-binding integrins to reinforce cell adhesion much before visible adhesion clusters are formed.