"Before the people of Chile, I do swear," Boric said as he took the oath in Valparaiso, the seat of Congress, for a four-year term replacing conservative leader Sebastian Pinera.
Boric takes over the reins of a country clamoring for change following mass protests in 2019, which he supported, against deep-rooted inequality in income, healthcare, education and pensions.
The revolt, which left dozens dead and hundreds injured, was the catalyst for a process now under way to rewrite Chile's dictatorship-era constitution.
Boric has vowed to relegate "to the grave" Chile's neoliberal economic model, which dates from the era of military despot Augusto Pinochet and is widely seen as sidelining the poor and working classes.
He has vowed to turn the country into a greener, more egalitarian "welfare state."
A lawmaker since 2014, millennial Boric inherits an economy ravaged by the coronavirus outbreak.
He has promised to introduce European-style social democracy to Chile, boosting taxes to pay for social reform, and all while putting the brakes on spiralling debt.
He will tackle these challenges with a cabinet comprised mainly of women and young people -- their average age is 42.
More than 20 international guests were confirmed to attend the investiture ceremony, including Alberto Fernandez and Pedro Castillo -- the presidents of neighboring Argentina and Peru -- King Felipe VI of Spain, and famed Chilean author Isabel Allende.