Scaffold proteins such as CNK1 are hubs for coordinating signalling pathways. Here, we demonstrate that CNK1 is a crucial transducer of growth factor-stimulated and oncogenic signalling. Activation of CNK1 depends on its dimerization executed by the N-terminal sterile motif alpha (SAM) domain. Accordingly, a CNK1 mutant lacking the SAM domain prevents CNK1-driven cell proliferation and matrix metalloproteinase 14 promoter activation. We identified phosphorylation of Ser22 by AKT as trigger for CNK1 dimerisation. Consistently, the mutant CNK1S22D mimicking constitutive phosphorylation stimulates CNK1 signalling, whereas the phosphoylation-dead mutant CNK1S22A does not. Searching the COSMIC database revealed Ser22 as target for oncogenic activation of CNK1. The mutant CNK1S22D and the oncogenic mutant CNK1S22F form clusters in serum-starved cells comparable to clusters of CNK1 in growth factor stimulated cells. Light-activatable CNK1, optoCNK1, based on the light-induced oligomerisation of cryptochrome 2 confirms that dimerization is the trigger for CNK1 activation. CNK1 dimers induced by activating Ser22 mutants or by light enhance cell invasion and ADP ribosylation factors 1 signalling associated with tumour formation. Positive and negative feedback mechanisms regulates CNK1 dimerisation and in this way its activity. Oncogenic mutants of CNK1 support the positive feedback while escape from negative feedback regulation.