Bob Beckwith was the retired firefighter who stood beside President George W. Bush at Ground Zero days after the attack.
Mourners gathered in Baldwin on Long Island, where Beckwith lived his entire adult life.
The priest said Beckwith was not one to seek the spotlight, but he was someone who could be relied upon.
"He was always there, always the one. There was a shyness about him - sometimes a quirkiness about him," the priest said.
President Bush released a statement saying Beckwith's courage represented the defiant, resilient spirit of New Yorkers and Americans after 9/11.
Beckwith was 91.