A novel series of N-substituted-4-phenylphthalazin-1-ones 14a-g bearing different anilines at the N-2 of phthalazin-1-one scaffold via acetyl-flexible linker was designed and synthesized for the development of potential anticancer agents. Compounds 19a-g were synthesized by insertion of methylene (CH2) bridge at C4-position of 14a-g to provide a flexibility for the phenyl group. The newly synthesized compounds 14a-g and 19a-g were evaluated for their anti-proliferative activity against three human tumor cell lines HepG2 hepatocellular carcinoma, HT-29 colon cancer and MCF-7 breast cancer. In particular, HepG2 and HT-29 cancer cell lines were more susceptible to the synthesized derivatives. Compound 19d (IC50 = 1.2 ± 0.09 μM) was found to be the most potent derivative against HepG2 as it was 2.9 times more active than doxorubicin (IC50 = 3.45 ± 0.54) and sorafenib (IC50 = 3.5 ± 1.04 μM). Compounds 14e, 14g, 19d and 19g with IC50 = (3.29 ± 0.45), (3.50 ± 0.846), (1.20 ± 0.09) and (3.52 ± 0.70) μM, respectively, were found to be active candidates against HepG2 cancer cells. Compounds 14e, 14g, 19d and 19g were able to induce apoptosis in HepG2, this was assured by; the significant increase in the percentage of annexin V-FITC-positive apoptotic cells (UR + LR), the down-regulation of the anti-apoptotic protein Bcl-2 and the up-regulation of the pro-apoptotic protein Bax, in addition to boosting caspase-3 levels. Moreover, cytotoxicity evaluation of the newly synthesized compounds in HT-29 revealed that compounds 14e, 14f, 19e and 19f (IC50 = 3.05 ± 0.78, 4.02 ± 1.18, 3.68 ± 0.79 and 2.98 ± 0.47 μM, respectively) were more potent than doxorubicin (IC50 = 7.70 ± 1.78 μM).