The majority of non-small-cell lung cancers involve EGFR overexpression, but clinical response to EGFR kinase inhibitors is limited almost exclusively to cells bearing a set of recently identified EGFR mutants. These mutations render cells “oncogene-addicted” to EGFR signaling, such that they readily undergo apoptosis in response to low concentrations of EGFR kinase inhibitors. Compared to WT EGFR counterparts, lung cancer cells with mutant EGFR display increased basal activity of the receptor, prolonged ligand-mediated receptor activation, and altered downstream activation dynamics of several key signaling intermediates. We have previously shown that these alterations result in part from a defect in the rate of ligand-mediated endocytosis of EGFR mutants. Here, we extend that work and demonstrate how altered EGFR endocytosis impacts downstream signaling dynamics in several key pathways. Specifically, we show that while the activities of some pathways are enhanced as a result of mutation, the activities of others are impaired. By overexpressing the EGFR mutants in cell lines of interest and working with cell lines with an inducible switch for the expression of a key protein involved in EGFR endocytosis, we have demonstrated that the alterations we observed in human lung cancer cell lines can be recapitulated in predictable ways and that they are tightly tied to receptor trafficking. In addition, we have shown that RNAi-mediated down-regulation of signaling intermediates impaired downstream of EGFR mutation in cells with WT EGFR increases cellular sensitivity to kinase inhibitors. In total, our results demonstrate that the EGFR mutations under consideration create a quantitative bifurcation of downstream signaling pathways, and that it is the net effect of these bifurcated signals that ultimately determines cellular response to EGFR kinase inhibition. Characterization of this bifurcation portends enhanced ability to successfully intervene pharmacologically in lung cancers with and without EGFR mutation.