Creighton had worked at i-level from 2003 until financial problems resulted in its liquidation in May.
Reporting to Mindshare's chief executive, Jed Glanvill, Creighton will focus on the agency's product delivery and creating new ways of integrating digital across divisions.
Ita Murphy, the managing director of Mindshare, will continue to focus on client relationships and people issues within the agency.
Mindshare is also looking to hire a replacement for Andy Zonfrillo, the former leader of its exchange division, who left to join Group M as its global head of pricing last month.
Creighton, who before i-level worked at the digital agency digit-all, will join Mindshare in September.
Glanvill said: "Mark has a clear view and belief in the potential of an integrated modern media agency."
At i-level, Creighton had day-to-day responsibility for the agency and was instrumental in its diversification into areas such as social media via its Jam division.
Creighton told Campaign that he opted to join Mindshare as it is "embracing the opportunities created by consumers' changing relationship with media.
"The ambition is to make Mindshare an invaluable business partner that redefines the role played by the modern media agency."