Lord Melchett was arrested on a GM protest
He stood down after being criticised for accepting a part-time consultancy with public relations firm Burson-Marsteller, whose clients include the GM food giant Monsanto.
In his new role he is expected to head a committee advising businesses on how to deal with controversial issues such as GM food, toxic waste and child labour in the Third World.
He has defended his decision to work for Burson-Marsteller, which has also handled the PR for companies in trouble over environmental incidents.
On the inside
These include Union Carbide after the Bhopal gas leak which killed up to 15,000 people in India.
He said: "I am interested in change for the better, change in government policy and practice and change in commercial - corporate - practice that benefits the environment.
"If I can use my experience to give people advice about what I think should happen - and, of course, I have not changed my mind about any of that - if there is the hope, and maybe occasionally the actuality, of change happening, then that is to the good."
No company had used such discussions with Greenpeace to pretend they were acting in a more environmentally friendly way than they actually were, the peer said.
Green disappointment
"I am happy to tell people, just as I did for 15 years with Greenpeace, what I think is right for the environment," he added.
But his move was met with disappointment by other environmental campaigners.
Mike Woodin, of the Green Party, said Lord Melchett was a man of "very high principles" but that it was difficult to believe that he thought he could change the company's "reprehensible" ethos.
Lord Melchett retired as executive director of Greenpeace in 2000, but remained on the board.
During his time there it grew from an organisation with a turnover of £1.4m, 12 staff and 25,000 supporters to a turnover of £7.5m, 80 staff and more than 200,000 supporters.