The acquisition of the search specialist agency will help McCann to enhance search engine optimisation capability for its UK agencies.
McCann Manchester will operate and manage Lakestar, with the search company continuing to manage its own client base, while working alongside McCann Worldgroup to deliver SEO to its clients.
Lakestar was founded five years ago by Demetrious Loizou, who is its head of sales. It has offices in London and Manchester, employing 50 people.
Neil McKay, chief executive of Lakestar, will continue to run the agency and the current management team will remain in place.
Lakestar's clients include Butlins, Halfords and The Co-operative.
Sue Little, chief executive of McCann Manchester, and Martin Jackson, chief financial officer of McCann Worldgroup UK, will join the board of Lakestar to manage the integration and development of the company within McCann Worldgroup.
Little said: "The completion of this deal significantly enhances our SEO offering and makes our overall integrated digital offering incredibly strong.
"We’ve wanted to grow our SEO offering for some time and we’ve been looking for a potential partner for well over a year."
McCann Worldgroup acquired interactive design agency All of Us in August last year to strengthen the group’s offering in the UK.
Follow Sarah Shearman on Twitter @Shearmans