A new series of 1,2,4-triazole-linked urea and thiourea conjugates have been synthesized and evaluated for their in vitro cytotoxicity against selected human cancer cell lines namely, breast (MCF-7, MDA-MB-231), lung (A549) prostate (DU145) and one mouse melanoma (B16-F10) cell line and compared with reference drug. The compound 5t showed significant cytotoxicity on MCF-7 breast cancer cell line with a IC50 value of 7.22 ± 0.47 µM among all the tested compounds. Notably, induction of apoptosis by compound 5t on MCF-7 cells was evaluated using different staining techniques such as acridine orange/ethidium bromide (AO/EB), annexin V-FITC/PI, and DAPI. Further, clonogenic assay indicates the inhibition of colony formation on MCF-7 cells by compound 5t. Moreover, the flow-cytometric analysis also revealed that compound 5t caused the arrest of cells at G0/G1 phase of cell cycle. In addition, the compounds when tested on normal human cells (L-132) were found to be safer with low cytotoxicity profile.