Protrusion of lamellipodia and ruffles requires polymerization of branched actin filaments by Arp2/3 complex. Although regulation of Arp2/3-complex activity has been extensively investigated, the mechanism of initiation of lamellipodia and ruffles remains poorly understood. Here we show that mDia1 acts in concert with the Arp2/3 complex to promote initiation of lamellipodia and ruffles. We find that mDia1 is an EGF-regulated actin nucleator involved in membrane ruffling using a combination of knockdown and rescue experiments. At the molecular level, mDia1 polymerizes linear actin filaments activating the Arp2/3 complex and localizes within nascent and mature membrane ruffles. We employ functional complementation experiments and optogenetics to show that mDia1 cooperates with the Arp2/3 complex in initiating ruffles. Finally, we show that genetic and pharmacological interference with this cooperation hampers ruffling and cell migration. Thus, we propose that the lamellipodium/ruffle-initiating machinery consists of two actin nucleators that act sequentially to regulate membrane protrusion and cell migration.