Myelin is a biologically active membrane receiving and processing signals from axons. Although much is known about its structure and molecular composition, the intracellular signal transduction pathways, active during specific phases of myelinogenesis for regulating myelin formation, remain poorly understood. Recent genetic loss-of-function studies have suggested a key role of extracelluar signal-regulated kinases-1 and -2 (ERK1/2), downstream mediators of mitogen-activated protein kinases (MAPKs), in promoting CNS and PNS myelination. In contrast, other studies, largely in vitro, have suggested that activation of ERK1/2 pathway can be detrimental for glial cell function and myelination. Given these conflicting reports, we investigated the effects of cell-autonomous activation of ERK1/2 in glial cells during developmental myelination in the intact CNS and PNS. Two lines of transgenic mice with sustained activation of ERK1/2 in oligodendrocyte progenitors (OPCs), oligodendrocytes, and Schwann cells were generated. Consistent with our loss-of-function studies, gain of ERK1/2 function in oligodendrocyte-lineage cells significantly increased myelin thickness, independent of oligodendrocyte differentiation or initiation of myelination. Additionally, increased activation of ERK1/2 in OPCs during early development resulted in transient hyperproliferation and overproduction of OPCs but generation of normal numbers of myelinating oligodendrocytes. Thus, these in vivo studies suggest a beneficial biphasic requirement of ERK1/2 during developmental myelination in the CNS, deployed first during early stages of the oligodendrocyte lineage for promoting OPC expansion and then redeployed later in myelinating oligodendrocytes for promoting myelin growth. Furthermore, Schwann cells with activated ERK1/2 hypermyelinate PNS axons, suggesting that ERK1/2 signaling is a conserved mechanism that promotes both CNS and PNS developmental myelination.