Born in Chicago on September 14, 1955, the Vatican says the pontiff will lead the new chapter with "peace, unity, and the defence of human dignity."
"Chicago's pope" was raised in south suburban Dolton and is a well-known White Sox fan.
SEE ALSO | Pope Leo XIV gifted Chicago White Sox pope hat
His childhood home at 212 E. 141st Place in Dolton is an official historical landmark. The Prevost family lived in the home until 1969.
It's not clear how the Chicago native celebrated his 71st birthday.
However, he visited the Vatican Library on Monday to launch an exhibit on water called "AQVA, Catastrophe and Wonder."
Photos show the pope viewing the installation "Diluvium" by French artist Jean Rene, known as JR.
The exhibit will open to the public from Sept. 25 through May 14, 2027.
For his 70'th birthday, he received a Portillo's chocolate cake.
SEE ALSO | Pope Leo receives Portillo's cake for 70th birthday; party held outside his childhood home in Dolton
AP News contributed to this report.