The TAL1 oncogene driving T cell lymphoblastic leukemia is frequently activated through mutated cis-regulatory elements, whereby small insertions or deletions (indels) create a binding site for the transcription factor MYB. Unraveling how non-coding mutations create oncogenic enhancers is key to understanding cancer biology and can provide important insights into fundamental mechanisms of gene regulation. Utilizing a CRISPR-Cas9 screening approach, we identify GATA3 as the key transcriptional regulator of enhancer-mediated TAL1 overexpression. CRISPR-Cas9 engineering of the mutant enhancer reveals a tandem GATA3 site that is required for binding of GATA3, chromatin accessibility, and MYB recruitment. Reciprocally, MYB binding to its motif is required for GATA3 recruitment, consistent with a transcription factor cooperativity model. Importantly, we show that GATA3 stabilizes a TAL1-MYB interaction and that complex formation requires GATA3 binding to DNA. Our work sheds light on the mechanisms of enhancer-mediated oncogene activation, where key transcription factors cooperate to achieve maximal transcriptional output, thereby supporting leukemogenesis.