We identified mutations in the IL7Ra gene or in genes encoding the downstream signaling molecules JAK1, JAK3, STAT5B, N-RAS, K-RAS, NF1, AKT and PTEN in 49% of patients with pediatric T-cell acute lymphoblastic leukemia (T-ALL). Strikingly, these mutations (except RAS/NF1) were mutually exclusive, suggesting that they each cause the aberrant activation of a common downstream target. Expressing these mutant signaling molecules—but not their wild-type counterparts—rendered Ba/F3 cells independent of IL3 by activating the RAS-MEK-ERK and PI3K-AKT pathways. Interestingly, cells expressing either IL7Ra or JAK mutants are sensitive to JAK inhibitors, but respond less robustly to inhibitors of the downstream RAS-MEK-ERK and PI3K-AKT-mTOR pathways, indicating that inhibiting only one downstream pathway is not sufficient. Here, we show that inhibiting both the MEK and PI3K-AKT pathways synergistically prevents the proliferation of BaF3 cells expressing mutant IL7Ra, JAK and RAS. Furthermore, combined inhibition of MEK and PI3K/AKT was cytotoxic to samples obtained from 6 out of 11 primary T-ALL patients, including 1 patient who had no mutations in the IL7R signaling pathway. Taken together, these results suggest that the potent cytotoxic effects of inhibiting both MEK and PI3K/AKT should be investigated further as a therapeutic option using leukemia xenograft models.