Expression of Lck, a T‐cell lineage‐specific tyrosine kinase critical for T‐cell development and activation, can be mediated by either proximal or distal lck promoter. We generated BAC transgenic mice in which BAC lck promoter was deleted and bred these transgenes to an Lck knockout background. Lck‐PROX mice, in which only the proximal promoter is functional, have maximal Lck protein and normal thymic development through CD4⁻CD8⁻ double negative (DN) and CD4⁺CD8⁺ double positive (DP) stages, but undetectable Lck later in development and reduced mature single positive thymocytes. In contrast, Lck‐DIST mice, in which only distal promoter was functional, are deficient in Lck protein in DN and DP thymocytes and severely defective in early T‐cell development, with a block at the DN3‐DN4 beta checkpoint equivalent to complete Lck knockouts. The ability of the proximal lck promoter to support thymic development is independent of Fyn; while, in contrast, the distal lck promoter alone is completely unable to support development in the absence of Fyn. Notably, normal thymocyte development is restored by presence of both proximal and distal promoters, even when independently expressed on different lck genes. These results define distinct and complementary requirements for proximal and distal lck promoters during T‐cell development.