Long will join the digital shop by the end of the year and replaces Matt Adams, who was named UK chief executive of Havas Media in June.
As head of digital at Carat since 2013, Long led a team of 35 specialists, working across global and domestic client accounts.
Long joined Dentsu Aegis Network in 2008 as Vizeum’s digital manager. She left for a stint at Microsoft in 2010 but rejoined Vizeum later that year as digital business director. Long was part of the team that launched Amplifi, Dentsu Aegis Network’s media buying arm.
Earlier this week, Dentsu Aegis Network named Ruth Stubbs as global president of iProspect, succeeding Ben Wood, who left the agency last month. Stubbs joins the global brand board and will be based in Singapore.