Epidemiological studies indicate that components of garlic (Allium sativum) have anti-proliferative effects against various types of cancer. In the present study, we investigated the effect of newly isolated phenylamine derivative N-benzyl-N-methyldecan-1-amine (NBNMA) from garlic cloves on the inhibition of the growth and apoptosis of human leukemia U937 cells and its potential anticancer mechanism. NBNMA exhibited an antiproliferative effect in U937 cells by inducing cell cycle arrest at the G2/M phase and apoptotic cell death. Western blot analyses revealed that NBNMA decreased the expression of the regulator genes of G2/M phase progression, cyclin dependent kinase (Cdk) 2 and Cdc2 and elevated the expression of the Cdk inhibitor p21WAF1/CIP1 in a p53-independent manner. In addition, NBNMA activated caspase-8 and caspase-9, initiator caspases of the extrinsic and intrinsic pathways of apoptosis, respectively, which led to activation of executioner caspase-3 along with degradation of poly(ADP-ribose) polymerase. NBNMA-induced apoptosis was observed in parallel with an increased ratio of pro-apoptotic Bax and Bad/anti-apoptotic Bcl-2 and Bcl-xL, and inhibition of inhibitor of apoptosis protein (IAP) family members XIAP and cIAP-1. Furthermore, NBNMA-treated cells displayed enhanced release of cytochrome c from the mitochondria into the cytosol concomitant with a loss of mitochondrial membrane potential and downregulation of Bid, suggesting that NBNMA-induced apoptosis occurred via the extrinsic and intrinsic apoptotic pathways with a possible link to Bid protein activity between the two pathways. These results indicate that NBNMA has promising potential to become a novel anticancer agent for the treatment of leukemia. We provide new insight into the mechanisms underlying the anticancer effect of NBNMA.