Thomas Collins, the great-grandson of Chicago Fire Department Lt. Patrick J. O'Donnell, said O'Donnell had been buried in an unmarked grave and officials had been unaware of the oversight, the Chicago Sun-Times reported Sunday.
O'Donnell's name is featured at the department's fire academy on a plaque honoring those who have fallen in the line of duty.
Family members researching their genealogy discovered the apparent mistake and went about finding an appropriate tombstone to honor their fallen family member, Collins said.
On Saturday, family members gathered at a cemetery in Evanston, Ill., to honor O'Donnell and his newly marked grave.
Collins said placing the phrase "We remember," along with the Chicago Fire Department logo, only seemed appropriate.
"We thought we owed it to the family," he told the Sun-Times. "My family has been a member of the Chicago Fire Department for 150 years."