Nick Suckley, who resigned as managing director of Media.com in July, and co-founders Pete Robins and Rhys Williams, have opened the doors to their Fitzrovia-based business.
Suckley said the agency would provide digital strategy and campaign planning for clients directly, and also through partnerships with other agencies.
"There is a massive demand from clients for something that goes beyond just an executional service," Suckley said. "A lot of the time, their questions are not being answered or thought about."
Agenda 21 intends to hire for both planning and support roles over the next few months as the agency wins new business.
Suckley said there was a skill shortage in digital media because traditional media planners had not got to grips with the medium, but many specialists were relatively junior and lacked experience in mainstream media.
He said Agenda 21 had the advantage of three co-founders who had worked in digital media between five and 10 years each, but also had traditional media experience.
Suckley and Robins have worked together since 1998, when they founded digital agency Media 21. They sold the business to MediaCom in 2000 and ran it as Beyond Interactive for one year before it was merged with Media.com.
Robins said the pair stayed at MediaCom as long as they did because it was a "fantastic agency", but after five years, felt it was time to run their own business again.
The trio then resigned from MediaCom in July and have been serving gardening leave and notice requirements since then.
Robins was media director at Media.com, while Williams, who joined the division from Media- Com Manchester in 2001, was associate director.
Kirsti Wilson, formerly head of interactive at BT, replaced Suckley as MD.