While Ras/mitogen-activated protein kinase (MAPK) signaling is activated in most human cancers, attempts to target this pathway using kinase active site inhibitors have not typically led to durable, clinical benefit. To address this shortcoming, we sought to test the feasibility of an alternative targeting strategy, focused on the ERK2 substrate binding domains, D and DBP. We found that disabling the ERK2-DBP domain in mice caused baseline erythrocytosis. Consequently, we investigated the role of the ERK2-D and -DBP domains in disease, using a JAK2-dependent model of polycythemia vera (PV). Importantly, inactivation of the ERK2-DBP domain promoted the progression of disease from PV to myelofibrosis (MF), suggesting that the ERK2-DBP domain normally opposes progression. ERK2-DBP inactivation also prevented oncogenic JAK2 kinase (JAK2V617F) from promoting oncogene-induced senescence (OIS) in vitro. The ERK2-DBP mutation attenuated JAK2-mediated OIS by preventing the physical interaction of ERK2 with the transcription factor, Egr1. Because inactivation of the ERK2-DBP created a functional ERK2 kinase limited to binding substrates through its D-domain, these data suggested that the D-domain substrates were responsible for promoting oncogene-induced progenitor growth and tumor progression, and that pharmacologic targeting of the ERK2-D domain might attenuate cancer cell growth. Indeed, pharmacologic agents targeting the ERK2-D domain were effective in attenuating the growth of JAK2-dependent myeloproliferative neoplasm cell lines. Taken together, these data indicate that the ERK-D and -DBP domains can play distinct roles in the progression of neoplasms and that the D-domain has the potential to be potent therapeutic target in Ras/MAPK dependent cancers.