Previously known as Cardinal Robert Prevost, Leo XIV was chosen as pope following a notably swift conclave last week.
He is the first US-born man to sit on the papal throne.
His brother John travelled to Rome from Chicago to celebrate with his brother after the election.
John Prevost told 9News that he had dinner with the new pope and shared Sunday mass with him at the tomb of Saint Peter in the Vatican.
“I think he’s relaxed. I think he’s kind of at ease,” Prevost said.
“I think he realises the responsibility that lays on his shoulders. And I think he will do just fine.”
Prevost said his brother “knows what he’s doing”.
During his inaugural Sunday noon prayers at Saint Peter’s Square, Pope Leo XIV appealed for peace in Ukraine and Gaza and urged for the release of Hamas’s remaining hostages.
Speaking from the balcony of St. Peter’s Basilica, Leo addressed roughly 100,000 onlookers, reiterating his plea to global powers: “never again war.”
Leo quoted Pope Francis in denouncing the number of conflicts ravaging the globe today, saying it was a “third world war in pieces.”
“I carry in my heart the sufferings of the beloved Ukrainian people,” he said. “Let everything possible be done to achieve genuine, just and lasting peace as soon as possible.”
As a bishop in Chiclayo, Peru, at the start of Russia’s 2022 full-scale invasion of Ukraine, then-Bishop Robert Prevost had not minced words in assigning blame to Moscow.
White smoke billows from Sistine Chapel chimney as new pope elected
According to a clip of a TV interview on the Peruvian show Weekly Expression, circulating in Italian media Sunday, Prevost said it was an “imperialist invasion in which Russia wants to conquer territory for reasons of power given Ukraine’s strategic location.”
In his remarks Sunday, Leo also called for an immediate ceasefire in Gaza, and for humanitarian relief to be provided to the “exhausted civilian population and all hostages be freed.”
Leo also noted that Sunday was Mother’s Day in many countries and wished all mothers, “including those in heaven” a Happy Mother’s Day.
