Weah becomes the 25th president of the west African nation.
George Weah emerged from Liberia's slums to become a superstar footballer in the 1990s, and has spent the last 13 years building political credibility to match his status as a sporting icon.
Weah, 51, put education, job creation and infrastructure at the centre of his policy platform to beat outgoing Vice President Joseph Boakai to the presidency, and expectations are sky-high he will deliver for the country's youthful population.
"I am a human being, I strive to be excellent, and I can be successful," Weah told journalists ahead of his inauguration, countering critics who say he is ill-prepared for office after serving in the Senate for just over three years.
The first African player to win both FIFA's World Player of the Year trophy and the Ballon d'Or, Weah was largely absent from Liberia during the 1989-2003 civil war period, playing for a string of top-flight European teams including Paris Saint-Germain and AC Milan, and later Chelsea.
"We love him, we just love him," said mature student Moses Adams, as he watched Weah and his veterans' team play a friendly game against the army ahead of his inauguration.
"I expect big change. I expect our lives to be better. I expect prices of things to go down, and I expect schools to be given priority," Adams said.
Arsenal boss Arsene Wenger has likened Weah's life to a film script, while the former striker has described Wenger as a "father figure" who stood by him as he faced racist attacks in Europe.
"I remember when I saw him for the first time in Monaco, coming in a bit lost, not knowing anybody, not being rated by anybody as a player and after, in 1995, becoming the best player in the world," Wenger said in January.
"Now he's president of his country -- it's an unbelievable story. But it's down to the fact that one thing that was common in George's attitude is being strong mentally, absolutely unbelievably convinced that he has a mission."
"The expectation of the people and the country is all up to him (Weah). Everybody believes that if he fails us, the majority will be disappointed with politics," said Samuel Harmon, 30, a street trader.
Critics however accuse Weah's Coalition for Democratic Change (CDC) of having too vague a political platform, and have challenged his long absences from the senate since being elected in a 2014 race he won over Sirleaf's son.
Weah is married to Clar Weah, and their son Timothy signed a professional football contract with Paris Saint-Germain in July 2017.