Chromosome instability is the most common genomic aberration, implicated in both cancer development, and malignant progression. Despite driving multi-drug resistance and metastasis; a details of the molecular origin of chromosome instability are still lacking. What is known, is that chromosome instability arises from aberrant mitotic division. Previous work by our lab identified that mitosis is under the control of MASTL kinase, an inhibitor of PP2A phosphatase. Loss of MASTL is embryonically lethal, while perturbation of its activity causes chromosome segregation errors, mitotic collapse, and cytokinesis failure. Therefore, we investigated whether MASTL has a role in the origin of chromosome instability, and thus oncogenic transformation. Not surprisingly, MASTL is commonly amplified or overexpressed in cancer. We show that high MASTL expression correlates with increased nuclear pleomorphisms in breast cancer, and poor patient survival in breast, ovarian and lung cancer. Overexpression of MASTL at biologically relevant levels in normal MCF10A breast cells delayed cell-cycle transit time, and increased the rate of chromatin bridges, suggesting that MASTL is directly involved in chromosome instability. These genomic aberrations were accompanied by a relief of contact inhibition and the induction of a partial epithelial-to-mesenchymal transition which are characteristics of oncogenic transformation. To determine the mechanism for MASTLs role in transformation we undertook a global phosphoproteomic analysis. This study identified that key members of desmosome assembly, stress-kinase signalling, and the actin-cytoskeleton were deregulated by MASTL overexpression. Importantly, knockdown of MASTL prevented invasion and metastasis of breast cancer cells in in vitro, organotypic, and intraductal mouse models. Taken together these results suggest that MASTL overexpression contributes to chromosome instability and metastasis and defines a novel role for MASTL as a regulator of epithelial to mesenchymal transition. This study provides a mechanistic basis for the action of MASTL in breast cancer, and its negative effects on survival and metastasis.