The Organization of American States (OAS) has picked Chilean minister Jose Miguel Insulza as its new leader.
Jose Miguel Insulza is seen as a tough negotiator
Following months of diplomacy, he was elected unopposed after a Mexican rival backed by Washington withdrew.
The top US diplomat for Latin America, Roger Noriega, said Washington would give Mr Insulza its full co-operation.
The growing number of left-wing states in South America supported Mr Insulza, but Bolivia and Peru refused to endorse the socialist interior minister.
Both countries have long-standing territorial disputes with Chile.
Mr Insulza built his reputation as a tough negotiator while Chile's foreign minister, the BBC's Clinton Porteous reports from Santiago.
In the late 1990s he led a successful fight for former military ruler Gen Augusto Pinochet to be returned to Chile, following his arrest in the UK.
As its new secretary general, Mr Insulza has pledged to invigorate the OAS, focusing on promoting democracy and addressing social issues.
But correspondents say that deep divisions remain among some of its members.
Mr Insulza and his rival, Mexican Foreign Minister Luis Ernesto Derbez, were deadlocked at 17 votes each after five ballots last month.
During the earlier voting rounds, the region appeared to be splitting along geographic lines.
Most of South America and the Caribbean supported the Chilean candidate, while Central and North America backed Mexico.
On Monday, Mr Noriega, the US Assistant Secretary of State for Western Hemisphere affairs, hailed the vote.
"Although today's outcome favoured Interior Minister Insulza, the true winner of this process is the organisation itself," Mr Noriega said.
"Count on the full co-operation of my delegation and the support of my government, achieving unity - democratic unity - and not giving space to facile claims that the hemisphere is divided along north-south lines," he said.
The post had been vacant since October, when former Costa Rican leader Miguel Angel Rodriguez quit, amid allegations of corruption.