Mullally, the first woman to serve as spiritual leader of the world's 85 million Anglicans, was welcomed by Leo, the first US leader of the world's 1.4 billion Catholics, into his formal office at the Vatican's ornate apostolic palace.
The two, who lead Christian denominations that split from each other in acrimony in 1534, met privately before going together to a 17th-century chapel where they recited prayers in unison.
In formal remarks to Leo, Mullally thanked the Pope for his new, forceful speaking style, which the pontiff used on a four-nation Africa tour to sharply denounce war and despotism and which attracted the ire of US President Donald Trump.
"The world needed this message at this time - thank you," the archbishop said on Monday.
"It reminded us that despite our sufferings, people long for life in all its fullness, and countless people are working each day for this vision of the common good."
Leo told Mullally that progress had been made in drawing the Catholic Church and Church of England together but lamented that "new problems have arisen in recent decades", without specifying those problems.
"We must not allow these continuing challenges to prevent us from using every possible opportunity to proclaim Christ to the world together," the Pope said.
Mullally, who is visiting Rome, was installed as the new Archbishop of Canterbury in March, in a historic shift that was met with mixed reactions from the global Anglican communion, particularly among more conservative provinces in Africa and Asia.
Before the meeting, Mullally told ITV News she felt "very humbled, very privileged" to meet Leo.
"There is a long relationship and fellowship between the Anglican Church and the Roman Catholic Church ... we will continue to build on that relationship," she said.
The Church of England broke away from the Catholic Church in 1534, sparked by Pope Clement VII's decision to refuse King Henry VIII's request for an annulment of his marriage to Catherine of Aragon.
The two denominations were strongly opposed to each other for centuries but have moved closer in recent decades.
Their teachings align on many major issues, though the Catholic Church does not ordain women and generally does not allow priests to marry.
Mullally promised in her remarks to the Pope that she would remain united with him in prayer.
"We receive from one another gifts we cannot generate alone: depth in prayer, courage in witness, perseverance in suffering, and faithfulness in service," she said.
King Charles III, the supreme governor of the Church of England, made a state visit to the Vatican in October 2025.
He and Leo prayed together in the Vatican's Sistine Chapel, in the first such joint worship including a pope and British monarch since Henry VIII's reign.
