Alberto Fernández assumed the presidency on Tuesday, returning the country to the ranks of left-leaning nations at a moment of right-wing resurgence in the Western Hemisphere.
In his inaugural speech to Congress as head of state, he pledged more aid for the poor and warned that the country would be unable to pay all its debts on time.
Taking the vice-presidency was Cristina Fernández de Kirchner, 66, a polarising figure who served as president from 2007 to 2015 and faces several charges of corruption from that time. Her presence has raised questions about the extent of her influence in the new administration.
Fernández, a 60-year-old lawyer, faces the immediate challenge of trying to pull Argentina from economic crisis while fulfilling promises of greater social justice. The country has a 35 percent poverty rate and is struggling to make debt payments on time. The economy is expected to shrink three percent by the end of 2019, with inflation at 55 percent.
"'I come before you to call for unity from all Argentina, to build a new social contract of brotherhood and solidarity," Fernández said in his inaugural address before Congress. "I come before you calling for all to put Argentina on its feet, to put the country on a path toward development and social justice."
He said that his administration's first meeting would focus on reducing hunger, and said that Argentina wanted to pay all its creditors but lacked the capacity to do so.
Outgoing leader Mauricio Macri became the first non-Peronist president to complete his term in 74 years, a landmark seen as a sign of Argentina's maturing democracy.
The two men embraced after Macri handed Fernández the presidential sash and staff, traditional symbols of leadership.
Peronists cheered the new president's arrival.
"I see a lot of people unemployed, a lot of hunger, and that is very frustrating," said Claudia Pouso, a 57-year-old retiree. I want everything to be turned around, more jobs for people. My daughter works in the hospital and there is nothing there. ... Everything needs to change."
The leader president has already announced plans to fight poverty with the distribution of subsidised basic foods, and he has outlined measures to lower food prices and fight malnutrition in poor families.
He has also announced plans to raise retirees' pensions and increase benefits for public employees and welfare recipients.