Cyclin-dependent kinase 4 (CDK4) plays a pivotal role in cell cycle regulation and is a well-established target in cancer therapy. Triazolopyrimidines, as bioactive heterocyclic compounds, represent a promising scaffold for the development of novel anticancer agents. Herein, a new series of 1,5-dihydro-[1,2,4]triazolo[4,3-a]pyrimidine derivatives (5a–g) is synthesized via multistep reactions involving 6-methyl-4-phenyl-2-thioxo-1,2,3,4-tetrahydropyrimidin-5-yl propionate and hydrazonoyl halides. Structural confirmation is achieved through infrared spectroscopy, 1H nuclear magnetic resonance, mass spectrometry, and elemental analysis, and further supported by alternative synthetic approaches. Molecular docking studies targeting the CDK4/cyclin D1 complex (PDB ID: 2W9Z) reveal favorable binding interactions, particularly for compounds 5c and 5d, with binding energies of −7.34 and −7.25 kcal/mol, respectively. In vitro cytotoxicity assays against HepG2 liver cancer cells show that compounds 5c, 5d, and 5f exhibit potent activity, with IC50 values of 4.38, 3.96, and 3.84 µM, respectively, comparable to doxorubicin (3.43 µM). A similar trend is observed in MCF-7 breast cancer cells, where 5c, 5d, and 5f again demonstrate strong antiproliferative effects with IC50 values of 4.12, 3.87, and 3.95 µM, respectively, close to doxorubicin (3.25 µM). The absorption, distribution, metabolism, excretion, and toxicity profile indicates excellent absorption, moderate distribution, low toxicity, and favorable drug-likeness. These findings highlight the potential of the synthesized triazolopyrimidine derivatives as promising leads for CDK4-targeted anticancer drug development.