Two natural ent-kaurene diterpenoids, ent-15α-angeloyloxykaur-16-en-3β-ol (7) and ent-15α-angeloyloxykaur-16-en-3β,9-diol (8), were extracted from the aerial parts of Distichoselinum tenuifolium, and six new derivatives were synthesised from compound (7). The antitumour properties of these natural and derivative ent-kaurenes (2, 7, 9–13) were evaluated in three cancer cell lines: HT29 (colon cancer), HepG2 (hepatocellular carcinoma), and B16-F10 (murine melanoma). Among them, the synthesised ent-kaurene (13) containing an exomethylene–cyclopentanone moiety showed the strongest antiproliferative effects in all cell lines tested, with significantly lower IC50 values around 2.5 μM. Compounds 13 and 12, together with their precursor (7), were selected for further comparative cytometric and microscopic analyses. Cell cycle studies revealed that derivatives 12 and 13 exhibited promising cytostatic activity by inducing selective G2/M phase arrest, particularly effective in HT29 and HepG2 cells. Conversely, precursor (7) showed no significant effect on B16-F10 cell cycle distribution. The Annexin V-FITC/PI double staining assay confirmed the robust apoptotic effects of compounds (7), 12 and 13, with compound 13 inducing up to 99% total apoptosis and exhibiting significant apoptotic activity in all cell lines tested. These apoptotic effects were closely linked to mitochondrial dysfunction, as evidenced by a marked loss of mitochondrial membrane potential and reduced Rh123 fluorescence in treated cells, thereby activating the intrinsic apoptotic pathway. These findings highlight the critical role of mitochondrial disruption in the cytotoxic mechanisms of these ent-kaurenes and underscore their potential as promising anticancer agents.