In the same week as the 60th anniversary of D-Day, 321 of the 523 regional branches that attended the charity's annual conference voted in the changes.
A new board of 13 elected and six appointed trustees will be chosen at the annual conference next June, and the transformation will be completed by late 2006.
Ian Townsend, the Legion's secretary-general, said the changes aimed to encourage other ex-servicemen's charities to come under the wing of the charity's revised structure in exchange for a seat on the board.
The vote came as part of the charity's eight-year 'Taking the Legion Forward' programme, and Townsend called the vote a "defining moment for the Legion".
"We will enforce a revised governance structure to mirror the needs of an organisation with an annual £70m turnover," he said.
The changes follow recommendations from the Combined Code on Corporate Governance created by the Financial Reporting Council last year and anticipate legislation that might come in the new Charities Bill.
He said this would replace "past reforms that left us with a decision-making process that relied heavily on the elected members of the council being guided by the wishes of our annual conference. To some extent we were stuck in a timewarp that mirrored the governance processes of the 1950s."
Townsend said 32 per cent of the Legion's 500,000 members had responded to the consultation process, which demonstrated "an overwhelming feeling for modernisation".
Trustees will only be able to serve three terms of three years, unlike the current council, whose members can be voted in an unlimited number of times.
Six trustees will be appointed from the business world, while the remaining spot will go to the chair of the women's section, previously only given a non-voting seat on the council.
Bureaucratic layers such as the intermediary management board will be disbanded and the post of general treasurer will be abolished to give trustees financial and strategic power.
Additionally, a membership council will be drawn from 10 regions to give members a clearer voice and an advisory assembly will act as a voice for the public and other stakeholders.