Lunatic Fringe (Lfng) enhances Notch1 activation by Delta-like 4 (DL4) to promote Notch1-dependent T-lineage commitment of thymus-seeding progenitors. Subsequently, Notch1 and T-cell receptor-β (TCRβ)-containing pre-TCR complexes signal CD4/CD8 double-negative 3 (DN3) committed T-cell progenitors to survive, proliferate, and differentiate into CD4/CD8 double-positive (DP) αβ T-cell precursors. Few DP thymocytes develop without Notch1 or pre-TCR signals, whereas ectopic Notch1 activation causes T-cell leukemia. However, mechanisms of a Notch-pre-TCR collaboration during this "β-selection" process are poorly understood. We genetically manipulated Lfng to attenuate or enhance Notch1 activation in DN3 thymocytes without inducing leukemogenesis. We show that Lfng temporally sustains DL-induced Notch1 signaling to prolong proliferative self-renewal of pre-DP thymocytes. Pre-TCR signaling greatly augmented Notch trophic functions to promote robust proliferation of pre-DP progenitors. In contrast, in the absence of DL/Notch signaling, pre-TCR-expressing progenitors rapidly atrophied and differentiated into DP thymocytes. Thus, Lfng prolongs Notch1 signaling to promote self-renewal more than differentiation during the early stages of β-selection. Our data provide novel insights into the Notch-pre-TCR collaboration, and suggest that decreasing Lfng expression during the DN3-DP transition minimizes the potent leukemogenic potential of Notch1 signaling.