Millard will "relocate to London" in mid-May but will "retain her current responsibilities" as vice-chair of US-based MediaLink "while she establishes the UK-based operations".
MediaLink, an advisory business that runs agency pitches and does headhunting, is expecting to make local hires in London office imminently.
Kassan, who founded MediaLink in 2003 and is visiting London this week with Millard, said: "Our plan is to go global and go fast."
He said Millard "has the global dexterity and business acumen required to expertly serve our clients in the UK and European markets as we continue to expand".
She has been previously a chief sales officer at Yahoo, with responsibilities including Europe, and was a founding member of the executive team at DoubleClick and helped to launch it in Europe.
Ascential, the London-based owner of Cannes Lions, bought MediaLink in February in a deal that values the business at up to $207m (£162m) if Kassan’s team hits targets.
Duncan Painter, chief executive of Ascential, said: "This is a clear step to fast-track MediaLink's business and further increase its presence outside the US. We are continuing to develop synergies between MediaLink and our other businesses to better serve our customers and bring additional elements to our growth strategy."
Kassan has looked at acquiring UK businesses. Prior to its sale to Ascential, MediaLink talked to headhunters The Lighthouse Company and Grace Blue and pitch intermediary Oystercatchers, which was bought by Centaur Media.
MediaLink also plans to launch in Asia.