Theodorou, whose previous roles also include managing partner at Addiction London before its acquisition by Mission, sits within the new creative services team at MHP + Mischief.
He will be responsible for driving digital innovation across the client portfolio and advise clients on new ways of communicating in the restrictive environment created by the coronavirus pandemic.
During his career, Theodorou has developed digital strategies for clients including B&Q, VOSS, Coca-Cola, Nomad Foods, Royal Mail, Post Office, KPMG and BAE Systems.
He will report to Gemma Irvine, head of creative services.
Engine-owned PR agencies MHP and Mischief moved into shared office space earlier this year – prior to the lockdown – in the culmination of a process of integration announced in 2019.
"Mario’s arrival is a big moment for us," said Alex Bigg, chief executive of Engine MHP + Mischief. "He’s the first major addition to our newly formed creative services team, which sits at the heart of our business and serves all of MHP and Mischief’s clients. Mario’s blend of strategic consultancy and creative ethos makes him a perfect addition to our team."
Theodorou said: "I’m thrilled to have joined such a progressive and innovative agency group. We have a shared belief in the power of digital storytelling and the importance of behavioural science. I’m looking forward to working with all the teams, and clients, to see where I can add value and help push the boundaries of our work."
His appointment follows the departure earlier this year of senior Mischief execs Gemma Moroney and Damon Statt, who held the roles of head of insight and strategy and executive creative director, respectively.
Engine MHP + Mischief made its debut as a combined entity on the PRWeek UK Top 150 Consultancies last week. It ranked in 18th place, with revenue of £26.4m in 2019 and 195 employees.