Though Gallucci will not be directly replaced as CEO, Dominic Shales has been promoted to the vacant role of MD and will lead the Next 15 Group agency alongside chairman and former ad man Stephen Woodford.
Woodford, who joined last winter, told PRWeek that Gallucci was leaving Lexis to set up a marketing software company.
He said that the departure of Gallucci, who took on the CEO role in 2011 following the resignation of Margot Raggett, had been a "carefully managed process internally", adding:
"Jason has been working on this idea for some time and when I joined making this transition was on the cards."
Gallucci departs amid moves by Lexis to diversify, with the agency recently hiring Red Bull's Tessa Barrerra to lead its social offer.
Woodford, who denied Gallucci's departure was linked to the changes, said: "It's about trying to broaden our capabilities so we can become a more integrated partner to our clients.
"PR is at the core of what we do, but in a world where earned media is becoming ever more important PR agencies are as well placed to be integrated brand leaders as advertising agencies have been, and you should expect to see more of that from us over the coming months."
Shales steps into the role having been head of digital, planning and content following the acquisition of his agency Paratus Communications in 2012.
Next 15 Group CEO Tim Dyson said: "We extend our thanks to Jason Gallucci, for the work he’s done in repositioning Lexis as an earned-media agency driving marketing success for its clients through the power of recommendation.
"We wish him all the best for the future. But I’m excited by the new leadership of Lexis. Stephen Woodford brings deep marketing experience that extends into the paid and owned arena through his work with DDB, while Dominic Shales brings strong digital capability. Putting this mix of skills at the top of the agency is a clear sign of where we see the future for Lexis."