Notch signaling affects a variety of mammalian stem cells, but there has been limited evidence that a specific Notch molecule regulates adult stem cells. Recently, it was reported that the reduced Notch signaling initiated at the embryonic stage results in a gradual hair graying phenotype after birth. Here we demonstrate that the oral administration of a γ‐secretase inhibitor (GSI) to wild‐type adult C57/Bl6 mice led to a gradual increase in gray spots, which remained unchanged for at least 20 weeks after discontinuing the GSI. In GSI‐treated mice, there was a severe decrease in unpigmented melanocytes in the bulge/subbulge region where melanocyte stem cells are located. While we confirmed that Notch1+/−Notch2+/− double heterozygous mice with a C57/Bl6 background were born with a normal hair color phenotype and gradually turned gray after the second hair cycle, in the c‐kit mutant Wv background, Notch1+/− and Notch2+/− mice had larger white spots on the first appearance of hair than did the Wv/+ mice, which did not change throughout life. Notch1+/−Notch2+/−Wv/+ mice had white hair virtually all over the body at the first appearance of hair and the depigmentation continued to progress thereafter. Using a neural crest organ culture system, GSI blocked the generation of pigmented melanocytes when added to the culture during the period of melanoblast proliferation, but not during the period of differentiation. These observations imply roles of Notch signaling in both development of melanocyte during embryogenesis and maintenance of melanocyte stem cells in adulthood, while the degree of requirement is distinct in these settings: the latter is more sensitive than the former to the reduced Notch signaling. Furthermore, Notch1 and Notch2 cooperates with c‐kit signaling during embryogenesis, and they cooperate with each other to regulate melanocyte homeostasis after birth.