The cancer diagnosis for a pontiff known to be relatively young and energetic and who was elected only in May 2025 caused shock.
A prayer was recited for Leo to have a good convalescence and complete healing during an open-air mass in St Peter's Square on Wednesday following the surgery.
The Pope was diagnosed on Tuesday with lung adenocarcinoma, the most common form of lung cancer, after doctors removed the lower lobe of his right lung during an operation.
Leo spent a peaceful night in hospital, the Vatican said, adding that further information about his condition would be released later on Wednesday.
The pontiff had been admitted to Rome's Gemelli hospital after doctors discovered a growth during a recent routine health check.
The operation lasted about 90 minutes and was completed without complications, the Pope's lead surgeon, Stefano Margaritora, and his personal physician, Luigi Carbone, said late on Tuesday.
The doctors said in a joint statement that the entire lower lobe of the right lung was removed in an effort to eliminate all cancerous tissue.
Human lungs have five lobes - three in the right lung and two in the left.
The Vatican has not disclosed the size of the tumour or whether the disease had spread to nearby lymph nodes, key factors in determining prognosis.
Leo is relatively young for a church leader, and enjoys sports, including tennis, and is not known to be a smoker or to have had recent health issues.
The Vatican said earlier this week that the Pope was expected to remain in hospital for about a week and that all his public engagements had been cancelled until October 20.
The mass on Wednesday was the closing act of an assembly of bishops called by Leo to discuss the church's family life teachings, the largest gathering of church leaders so far during this papacy.
Filipino Cardinal Luis Tagle led the mass in place of Leo.
The Pope last week approved Tagle as the dean of the Church's College of Cardinals, in the first selection of an Asian cleric to the elite role.
Political leaders offered messages of support.
US Secretary of State Marco Rubio said he and his wife Jeanette were praying for the pope, an American who was born in Chicago.
"May the Lord give him strength and the Holy Spirit guide the medical professionals taking care of the Holy Father," he said on X.
Rubio met Leo at the Vatican in May.
There was no immediate response on social media from US President Donald Trump, who has criticised the Pope, or Vice-President JD Vance, who is a Catholic.
European Commission President Ursula von der Leyen sent her best wishes.
"His serenity and strength are an inspiration for all of us. We hope for his full and speedy recovery," she wrote on X.
Leo usually follows a very busy schedule.
He is due to make a tour of Uruguay, Argentina and Peru from November 6 to 17, but it was not clear if this trip would still go ahead.
The Pope is the ultimate earthly authority for the world's 1.4 billion Catholics and does not have a deputy who steps in when he is sick.
When a pope is incapacitated, the business of the Vatican's vast bureaucracy carries on as usual, holding off on any decisions that require papal approval.
