The Saints, who have been celebrating the anointment of their most famous fan, won 1-0 against Colon despite being reduced to 10 men after their opponents handed them a victory with an own goal.
Colon forward Ruben Ramirez, helping out in defence, rose to clear a San Lorenzo corner kick with his head but managed instead to deflect the ball into his own net in the 74th minute.
San Lorenzo were a man short from the 32nd minute after central defender Mauro Cetto was shown a straight red card for dissent, having apparently insulted a linesman.
"Maybe we were helped by the luck of the Pope," San Lorenzo midfielder Juan Mercier told reporters.
Saints fans held up banners celebrating the Pope, a fan of the team since his childhood, who in 2008 held a mass at the club's premises to celebrate their centenary.
San Lorenzo, who owe their name to a priest, Father Lorenzo Massa, in the Almagro neighbourhood of Buenos Aires, are looking for their 11th Argentine league title. They won their last in 2007.
Following the defeat, Colon coach Roberto Sensini resigned after guiding the team to just two draws from six matches in the "Final" championship.