More than 200 different departments from around the country - and even Canada - arrived to pay their respects to the 61 year old.
"First responders are our better angels, our true guardians, those we entrust to safeguard our families. And as a city family, it is now our turn to safeguard the memory of firefighter Glenn Allen," said Mayor Antonio Villaraigosa.
The speakers remembered Allen as a quiet, hard-working man who loved airplanes, music and playing Sudoku. But most of all, he loved his blood family and his fire-fighting family.
"Glenn lost his life doing what he was called to do, to help others," said Chaplain Danny Leon during the funeral.
Services began at 9:30 a.m. at the Cathedral of Our Lady of the Angels in downtown L.A. following a procession from City Hall.
Friends and family honored Allen with a large candlelight vigil Thursday night at Fire Station 78 in Studio City.
"In my eyes, Glenn was the complete man. A family man, a role model, a leader, a friend," said LAFD Firefighter Jerry Updegraft.
Allen responded to a Hollywood Hills fire on Feb. 16. Crews didn't realize flames on the roof had melted a sprinkler system, filling it up with water. Authorities said the ceiling of the mansion collapsed on him and five other firefighters.
The 61-year-old firefighter was rushed to a hospital, but he died from his injuries two days later.
Allen, a 38-year veteran on the force, was about a year from retirement. He is survived by his wife, Melanie, and his daughter, who gave birth to his first grandson a day after he died.
Investigators returned to the 12,500-square foot mansion on Thursday. Even though firefighters have ruled out arson, the cause of the fire could still be considered negligence against the home owner. Authorities said the investigation could take weeks.
Allen was the first Los Angeles Fire Department firefighter to be killed in the line of duty since March 2008, when Brent Lovrien was killed in an explosion near Los Angeles International Airport as he investigated an earlier blast that blew manhole covers into the air.