McKechnie, who popularised the phrase 'rip-off Britain' during the height of the new car pricing debate in 2000, had led the group since 1995, campaigning for consumers' rights and safety.
Matthew Carrington, RMI chief executive, said he was “saddened” by the news.
“The Consumers' Association has lost an extremely formidable and well-respected advocate who worked tirelessly on behalf of the general public. Her contribution to raising customer standards in the motor industry was considerable, and the RMI was particularly appreciative of her advice and help in developing the new industry Code of Practice, CarWise,” he added.
Chancellor Gordon Brown, who knew McKechnie since University, praised her “unfailing support for great and noble causes”.