Cancer is a worldwide epidemic and a leading cause of death. There is a need to find natural solutions for therapy to prevent that outcome. Olive pomace is a known source of bioactive compounds. This work aimed to obtain an olive pomace extract, using an innovative green method, and to assess its chemical composition and the effect of its application in different cancer cell lines. The extract was composed of sugars (28%), minerals (11%), lipids (8%), mostly oleic acid (72%), protein (1%), phenolics (3 g gallic acid equivalents/100 g), hydroxytyrosol (215 mg/100 g), flavonoids (2 g catechin equivalents/100 g), and α-tocopherol (2 mg/100 g). Moreover, 68 phenolics were tentatively identified by UHPLC-ESI-QTOF-MS. The extract significantly reduced the proliferation of the studied cancer cell lines (breast: MCF-7; pancreatic: AsPC-1; and colorectal: HT-29 and Caco-2). It was able to reduce angiogenesis in AsPC-1 cells, presented a cytotoxic effect and increased malondialdehyde levels in HT-29 cells, and decreased growth of Caco-2 cells. These results suggest that this extract can be a source of several compounds which can be used in the formulation of dietary supplements or as a structural basis for antitumoral drugs, but their mechanistic actions still need to be assessed.