Pope Leo XIV's inaugural Mass in St. Peter's Square drew approximately 100,000 people, where he called for a “united Church.” During the inauguration, attended by around 200,000 people including cardinals and world leaders, the Pope promised to resist the "temptation to be an autocrat" and not to rule by "religious propaganda." He highlighted the prevalence of discord and wounds caused by hatred, violence, prejudice, and an economic paradigm that exploits resources and marginalizes the poor. He also paid tribute to his predecessors, Pope Francis and Pope Leo XIII.

The American pontiff emphasized that the Catholic Church's authority should not be about capturing others through force or religious propaganda, but rather governing the world's 1.2 billion Catholics without autocratic tendencies. He expressed a desire for a united church that serves as a leaven for a reconciled world.

Ukraine's President Volodymyr Zelenskyy met with Pope Leo XIV privately after the inaugural address, expressing that the Pontiff is a symbol of hope for peace and that the Holy See's authority could help end the war. Zelenskyy also met US Vice President JD Vance. The UK was represented by Prince Edward, Deputy Prime Minister Angela Rayner, and Foreign Secretary David Lammy. Peruvian President Dina Boluarte also attended, given Pope Leo's joint US-Peruvian citizenship.

The pontiff prayed at the tomb of St Peter and was presented with the lambswool stole (pallium) and the fisherman's ring. The pallium symbolizes the pastor carrying his flock, while the ring represents Jesus's call to Peter to cast his fishing net.

President of Nigeria, Bola Tinubu, commended Pope Leo XIV's humility, noting his history with Nigeria and message of peace as reminders of moral leadership. Tinubu affirmed Nigeria's commitment to promoting peace and unity across the world without bias. He also reiterated Nigeria’s dedication to fostering interfaith dialogue amid global tensions.

Pope Leo XIV formally began his reign by reaching out to conservatives, calling for unity, vowing to preserve the Catholic Church's heritage and not rule like "an autocrat." He was formally installed as the 267th pontiff of the Roman Catholic Church and sovereign monarch of Vatican City at an outdoor Mass.