Crowds are expected to cram the Square in the Vatican and surrounding streets in Rome for the formal celebration, which starts at 10am (local time) and includes the first ride in the white popemobile by Leo, the first pope from the United States.
Born in Chicago, the 69-year-old pontiff spent many years as a missionary in Peru and also has Peruvian citizenship, meaning he is also the first pope from that South American nation.
Robert Prevost, a relative unknown on the world stage who only became a cardinal two years ago, was elected pope on May 8 after a short conclave of cardinals that lasted barely 24 hours.
He replaces Pope Francis, from Argentina, who died on April 21 after leading the Church for 12 often turbulent years during which he battled with traditionalists and championed the poor and marginalised.
US Vice President JD Vance, a Catholic convert who clashed with Francis over the Trump administration's hard-line immigration policies, will lead a US delegation alongside Secretary of State Marco Rubio, who is also Catholic.
Ukrainian President Volodymyr Zelenskiy will also attend and would be happy to meet other leaders, a top aide has said, as he did at Francis' funeral when he had face-to-face talks with US President Donald Trump in St Peter's Basilica.
Zelenskiy last met Vance in February in the White House, when the two men clashed fiercely in front of the world's media.
Also expected at the Vatican ceremony are the presidents of Peru, Israel and Nigeria, the prime ministers of Italy, Canada and Australia, German Chancellor Friedrich Merz and European Commission President Ursula von der Leyen.
Many European royals will also be in the VIP seats near the main altar, including Spanish King Felipe and Queen Letizia.
Focus on peace
In various sermons and comments since his election as pope, Leo has praised Francis repeatedly but has not offered many hints about whether he will continue with the late pontiff's vision of opening the Church up to the modern world.
His homily on Sunday is likely to indicate some of the priorities for his papacy, having already made clear over the past 10 days that he will push for peace whenever possible.
His first words in an appearance to crowds in St Peter's Square on the night of his election were "Peace be with you all," echoing words Catholics use in their celebrations.
In a May 14 address to officials of the eastern Catholic Churches, many of whom are based in global hot spots such as Ukraine and the Middle East, the new pope pledged he would make "every effort" for peace.
He also offered the Vatican as a mediator in global conflicts, saying war was "never inevitable".
Sunday's Mass will feature prayers in several languages, in a nod to the global reach of the 1.4-billion member Church, including Latin, Italian, Greek, Portuguese, French, Arabic, Polish, and Chinese.
As part of the ceremony, Leo will also formally receive two items as he takes up the papacy: a liturgical vestment known as a pallium, a strip of lambswool which represents his role as a shepherd, and a special band known as the fisherman's ring.
The ceremonial gold signet ring is specially cast for each new pope and can be used by Leo to seal documents. It features a design of St Peter holding the keys to Heaven and will be broken after his death, marking an end to his papacy.
Pope expected to live at Apostolic Palace
Pope Leo is expected to live in the Vatican's Apostolic Palace, unlike Francis who shunned the palace in favour of a Vatican hotel, according to several Vatican officials.
The papal apartments occupy the top floor of the palace, a 16th century building that overlooks St Peter's Square. They feature about 10 rooms, including a bedroom, private library and study, dining room, kitchen and chapel.
Renovation works at the palace are under way and Leo was expected to move in some time after the Mass on Sunday, a senior Vatican official told Reuters, asking not to be identified.
Another official said the Vatican thought it would be easier to provide security for the new pope at the palace than at the hotel facility where Francis lived.
Leo has been living in an apartment that he previously used at the building that houses the Vatican's doctrinal office near St Peter's.
