Although no official winner had been declared on Sunday night, Mr Fernandez had 48.1 per cent of the vote compared to 40.4 per cent for Mr Macri with 97 per cent of votes reported.
Mr Fernandez’s election will also see the return of a divisive former president and first lady, Cristina Fernández de Kirchner, as his running mate.
Ms Kirchner, who won two presidential elections but also faced several corruption scandals between 2007 and 2015, told supporters on Sunday: “Today, Alberto is the president of all Argentines.”
Mr Fernandez is expected to meet the 45 per cent level of support needed to avoid a runoff vote with his nearest rival on 24 November.
The 60-year-old lawyer told the crowd he would need the support of Mr Macri’s administration to reconstruct what he called the inherited “ashes” of Argentina.
“The only thing that concerns us is that Argentines stop suffering once and for all,” Mr Fernandez said.
Earlier in the evening, Mr Macri told supporters he had called Mr Fernandez to congratulate his opponent and invited him for a breakfast chat on Monday at the presidential palace.
“We need an orderly transition that will bring tranquillity to all Argentines, because the most important thing is the well-being of all Argentines,” he said.
When he was elected in 2015, Mr Macri promised to boost Argentina’s economy, which is the second-largest in South America, but he saw its currency plunge during his presidency.
The country, which is one of the world’s top grains suppliers, has one of the highest inflation rates in the world and about one third of Argentines are in poverty.
However, the possibility of a return to the interventionist economic policies seen when Ms Kirchner was president has threatened to rattle financial markets.
Opponents of Mr Fernandez have claimed that Ms Kirchner is the real power behind his campaign.
Early on Monday, Argentina’s central bank said it would tighten currency controls by sharply limiting the amount of dollars people can buy from $10,000 to $200 per month by bank account and $100 in cash until December.
There are fears that a rush to buy dollars could see the Argentinian peso plunge again following Mr Macri’s defeat.
“The last two years have been brutal in Argentina,” Benjamin Gedan, an Argentina expert at the Woodrow Wilson International Centre for Scholars, said.
“Voters have suffered a painful recession, unimaginably high inflation and a debt crisis. No incumbent could survive in these conditions.”
Mr Gedan added that Mr Fernandez is “an untested leader” who is inheriting a ruinous economy and unfavourable international conditions.
“The government of Alberto Fernandez and Cristina Kirchner will feature Peronists of varied political tendencies, but the crucial question is what the dynamic will be between the pragmatic president and more ideological and polarising vice president,” Michael Shifter, the head of the Inter-American Dialogue, a Washington-based think tank, said.
“The nature of that power struggle will determine the direction of Argentina's economic, social and foreign policy in coming years.”
However, Mr Shifter did not believe a return to the populist policies of Ms Kirchner’s presidency was likely.
“Today, Argentina simply does not have the economic conditions for unchecked spending,” he said.
“This will not be a replay of her presidency.”
Additional reporting by AP
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