Crowds flocked to a cathedral in Harissa, north of Beirut, to attend the mass on Sunday.
The rally came two days after hundreds of thousands attended a demonstration in Beirut organised by the Shia Muslim militia, Hezbollah, to celebrate what it called its recent "victory" over Israel in the month-long conflict.
Geagea said: "We are the victors, and yet we do not feel it was victory but rather that a real catastrophe befell our country, and that our fate and destiny are at the mercy of the winds."
Supporters waved his picture, and the white, red and green flags of the Lebanese Forces as the party leader spoke at the shrine of the Virgin Mary in Harissa.
"We are the victors because it was us who were demanding the [Lebanese] army's deployment [in south Lebanon], backed by Unifil [peacekeepers], while they were opposed," he said without naming Hezbollah.
Geagea is a member of the anti-Syrian "March 14" parliamentary group that has criticised Hezbollah's capture of two Israeli soldiers on July 12 and has called for the group to disarm.
It was the first service he had attended after he was released from prison last year after serving more than a decade on multiple murder charges from the war.
Hezbollah chief Nasrallah wants
a new government in place
The former militia leader had not attended the annual mass for 12 years because of his imprisonment.
He was arrested in April 1994 and his group was banned, after a church bombing that killed 10 people.
Gegea was later acquitted in the bombing but sentenced to three life terms on several other murder counts, including the killing of Rashid Karami, the pro-Syrian prime minister in 1987.
Geagea served 11 years in prison before he was released in July 2005, after Lebanon's parliament approved a motion to pardon him.
He led the Lebanese Forces - the country's most powerful Christian militia during the Lebanese civil war.
Israel backed his militia during the war, and during the Israeli invasion in 1982 to expel Palestinian guerrillas.