Association of GATA-1 and its cofactor Friend of GATA-1 (FOG-1) is essential for erythroid and megakaryocyte development. To assess functions of GATA-1-FOG-1 association during mouse development, we used the GATA-1 hematopoietic regulatory domain to generate transgenic mouse lines expressing a mutant GATA-1, which contains a substitution of glycine 205 for valine (V205G) that abrogates its association with FOG-1. We examined whether the transgenic expression of mutant GATA-1 rescues GATA-1 germ line mutants from embryonic lethality. In high-expressor lines we observed that the GATA-11V205G rescues GATA-1-deficient mice from embryonic lethality at the expected frequency, revealing that excess GATA-1V205G can eliminate the lethal anemia that is due to GATA-1 deficiency. In contrast, transgene expression comparable to the endogenous GATA-1 level resulted in much lower frequency of rescue, indicating that the GATA-1-FOG-1 association is critical for normal embryonic hematopoiesis. Rescued mice in these analyses exhibit thrombocytopenia and display dysregulated proliferation and impaired cytoplasmic maturation of megakaryocytes. Although anemia is not observed under steady-state conditions, stress erythropoiesis is attenuated in the rescued mice. Our findings reveal an indispensable role for the association of GATA-1 and FOG-1 during late-stage megakaryopoiesis and provide a unique model for X-linked thrombocytopenia with inherited GATA-1 mutation.