Prime's executive creative director Tom Beckman joined Weber’s global leadership team after the deal, which came eight months after Lungu was appointed to her role.
Lungu told PRWeek that she had resigned but the decision was in no way related to the Prime acquisition.
She added: "Tom is a friend, and so is Colin; we know each other since 2010, when we judged at the Cannes Lions together. Getting to work together to strengthen the creative reputation of the firm in the region was actually a big plus of the last months. I'm really grateful for these very successful two years with the agency, but for me it's time for a new chapter."
Lungu joined Weber Shandwick in March 2013 as EMEA consumer practice leader. She was previously the managing partner and creative director of The Practice, the Romanian PR agency she founded in 2006.
Colin Byrne, EMEA chief executive of Weber Shandwick, said: "Gabriela has had a great two years with us – her first non-Romania, big PR agency role."
"She has been a real change agent, embedding and empowering creative teams across our network and rolling out a thirty office Creative Revolution training programme. We wish her well for her next role, in whatever discipline and country that may be."
This article was updated on 6 November with comment from Gabriela Lungu.