Pancreatic β- and α-cells play essential roles in maintaining glucose homeostasis. However, the mechanisms by which these distinct cell populations are generated, expand, and mature during pancreas development remain unclear. In this study, we addressed this critical question by performing a single-cell transcriptomic analysis of mouse β- and α-cells sorted from fetal to adult stages. We discovered that β- and α-cells use different regulatory strategies for their maturation and that cell proliferation peaks at different developmental times. However, the quiescent and proliferative cells in both the β-lineage and α-lineage are synchronous in their maturation states. The heterogeneity of juvenile β-cells reflects distinct cell cycling phases, origins and maturation states, whereas adult β-cells are relatively homogeneous at the transcriptomic level. These analyses provide not only a high resolution roadmap for islet lineage development but also insights into the mechanisms of cellular heterogeneity, cell number expansion, and maturation of both β- and α-cells.