Approximately 20% of breast cancers (BC) are characterized by the gene amplification and overexpression of HER2, a member of the ErbB/HER receptor family. While targeted therapies against HER2 effectively delay disease progression in this BC subtype, details of how overexpressed HER2s drive these tumors to malignancy are still unclear. To better understand this process, we investigated cellular responses to HER2 overexpression in individual live cells. We developed novel single receptor diffusion analyses to probe the spatial distribution of HER2s and determine their activation status by their diffusivity. Surprisingly, we found that HER2 overexpression induced deformation of basal cell membranes, which depended only on the HER2 density, regardless of the receptor signaling activity. Moreover, this membrane deformation lowered the focal adhesion coverage on the cell surface, which reduced cell adhesion and increased cell motility. These findings suggest that there is a signaling-independent role of HER2 overexpression in disease progression of HER2 positive BCs.