South America's largest trade bloc said in a statement that Venezuela would "help bring necessary balance" to the council.
Hugo Chavez, the Venezuelan president, has said that he wants his nation to join the council to dilute Washington's "imperialist" influence.
The US has promoted its ally Guatemala as a rival candidate to Venezuela for a seat on the council and Chavez has accused Washington of trying to persuade Latin American countries to vote against him.
Venezuela joined Argentina, Brazil, Paraguay and Uruguay as full members of Mercosur this month.
Bolivia and Chile are associate members.
With its new member, Mercosur now has a total population of more than 250 million people, a gross regional product of over a trillion dollars and regional trade surpassing $300 billion.
Venezuela's joining brought in the region's biggest oil producer and exporter and only member of oil cartel Opec.
During the two-day summit the group also backed proposals to establish a continent-wide free-trade pact.
Fidel Castro, the Cuban president, made a rare international appearance at the meeting.
He said: "Nobody knew I was coming, not even me."
He also said he was "very happy at turning 80, something that I never had hoped for, much less when we had a neighbour, by chance the most powerful force worldwide, trying to eliminate me every day".
Castro signed a complementary economic accord with the group which will benefit Cuba in its trade with Mercosur countries.
In a thinly veiled reference to the US, Castro said: "This kind of integration has centuries-old enemies."