A former TV journalist, Funes' peaceful background helped the Farabundo Marti National Liberation Front, or FMLN, win power through the ballot box for the first time on Sunday years after it fought one of the Cold War's nastiest conflicts.
Venezuela's socialist President Hugo Chavez was swift to send congratulations.
But rather than swing El Salvador to the hard left after 20 years of pro-Washington rule by the ARENA party, Funes said he wanted even closer U.S. relations to help tackle joint issues like migration, street gangs and drug smuggling.
"I would aspire to strengthen relations with (U.S. President Barack) Obama," he told Reuters Television after his win against ARENA's Rodrigo Avila late on Sunday, which sparked noisy street celebrations by red-clad supporters.
Funes also urged unity and reconciliation with the ARENA party, which was founded by an army major closely associated with right-wing death squads in the 1980-92 civil war.
Scars from the war mean the left-right breach runs deep, and it will take time to move on from a barbed election race where ARENA called the FMLN "communists" and "terrorists".
ARENA leaders were visibly smarting after their defeat.
"The country is totally divided," snapped ARENA's San Salvador director Adolfo Torres as he waded through a sea of dismayed supporters singing old campaign songs late on Sunday.
After years as a peaceful opposition party, the FMLN cashed in on electoral fatigue over ARENA's long rule and fears of the world economic crisis to win by nearly 3 percentage points, scotching the notion that old associations with Marxism and rebel warfare made the ex-rebels unelectable.
"(It) doesn't mean a leap into a vacuum nor a break with the system," said Funes, 49, who never fought in the civil war.
Financial Crisis
The Nationalist Republican Alliance, or ARENA, kept the coffee-exporting Central American country firmly in the pro-Washington camp as it held office since 1989, even sending small contingents of troops to help U.S. forces in Iraq.
But rampant poverty and street crime have helped the FMLN, which during the Cold War fought vastly better-equipped forces armed by the United States at a cost of billions of dollars. The guerrillas laid down their arms under a 1992 peace deal.
The nearest the rebels came to taking power was in late 1989 when they poured into the capital in an offensive that was only halted when the military bombed and shelled guerrilla positions in residential areas.
"This victory ... has cost years of fighting, sacrifice and blood," said FMLN lawmaker Orestes Ortez.
Funes said his first priority would be to tackle the economic crisis looming over El Salvador, a poor country with landscapes dotted with volcanoes, lakes and verdant hillsides.
About a quarter of El Salvador's population—some 2.3 million people—live in the United States and the money they send home is key to the economy. The U.S. recession has also dampened demand for El Salvador's factory exports, despite a regional free trade deal with the United States.
"There is no time to lose. From tomorrow we will start taking the necessary decisions," Funes said.
He told Reuters Television he hoped to model his economic policies on those of Brazil's moderate leftist President Luiz Inacio Lula da Silva, who was one of the first to telephone late on Sunday to congratulate Funes on his win.
Funes says he will crack down on tax evasion by the wealthy and use the funds to create jobs for Salvadorean immigrants returning from the United States. He also vows to invest in farming to reduce dependence on imported food.
Despite Funes's insistence he will be a pro-business moderate, his victory was a boost for more radical Latin American leaders like Chavez and Nicaragua's Daniel Ortega.
Vice President-elect Salvador Sanchez is a veteran FMLN hard-liner who many fear could take policy to the left.
Avila, a former national police chief, said his party would make sure El Salvador does not turn authoritarian under the FMLN. "We will be a constructive opposition, an opposition that is vigilant so that liberties are not lost in our country."
