Adams’ role will be to work with clients such as Haven Gateway Partnership, as well as developing a strategy for new business with Riverside managing director Dave McCullough.
McCullough said: "In two years we’ve grown to become a very successful consultancy. But with Mark’s experience in running his own consultancy and growing it into one of the best-known and profitable in the business, I really feel we will go to the next level."
Adams is currently chairman of the London Chamber of Commerce and was head of public affairs at Lansons Communications, which bought the company he founded, Foresight, in 2010.
A year after joining Lansons, Adams left the company to launch a campaign for higher professional standards in lobbying, called standup4lobbying.
Earlier in his career, Adams also spent six years as private secretary to the Prime Minister, working with John Major and Tony Blair.
Commenting on his new role, Adams said: "Having known Dave for a few years, we had often talked about working together. With Riverside at an exciting stage in its young life, we felt there was much we could offer each other in growing the business."
In February, The Guardian reported that Adams had lost a libel case to the former Conservative Party co-treasurer Peter Cruddas and was forced to pay £45,000 plus a proportion of Cruddas’ £120,000 legal bill for suggesting that the politician had breached the Political Parties, Elections and Referendums Act 2000.