France has suggested bringing emerging economies on board as members of the exclusive world club of G8 industrialised nations.
Mantega did not specify which emerging-market nations besides Brazil, Russia, India and China should be allowed to join.
European presidents on Friday suggested making the International Monetary Fund the world's financial watchdog, giving it more power to curb financial crises, with more money to aid countries in trouble.
But Mantega said Brazil believes that role should go to an expanded version of the G8, turning it into a group of nations numbering as many as 15.
While heads of state of the G20 nations will attend a financial crisis summit in Washington on November 15, the group typically has not been a power broker in determining important world economic decisions because it is led by finance ministers and central bank presidents.
"We are still directed and controlled by institutions that reflect the economic situation of the 1940s and 1950s," Mantega said.
Specifically, he said, the system put in place by the 1944 Bretton Woods agreement was outdated and needed to be changed to take into account the greater economic importance of emerging nations.
"We consider that the present institutions failed," Mantega said.
"They did not know how to avoid this financial problem. They did not detect it in time and failed to prevent it from happening.
"We could strengthen the G20, or the G7 could be expanded and house the main emerging-market countries."
Whatever group ends up being in charge, Mantega said nations like Brazil will no longer sit idly "as mere coffee drinkers" at summits while rich nations make important financial decisions.