Mr Mintz underwent seven hours of surgery and suffered two broken legs and significant blood loss from his injuries in the tragedy that left 10 dead, including the shooter.
The gunman burst into a classroom, shot a professor in the head and then ordered cowering students to stand up and state their religion before shooting them one by one, according to survivors' accounts.
As Mercer moved toward an adjoining classroom, Mr Mintz, an army combat veteran who served in Iraq, threw himself against the classroom door to prevent the gunman from entering.
But Mercer managed to blast his way in and repeatedly shot Mr Mintz, ignoring pleas that it was his son Tirek's sixth birthday.
"He gets shot three times, hits the floor, looks up at the gunman and says 'it's my son's birthday' and gets shot more times," Mr Mintz's aunt, Wanda Mintz, told local television state Fox 8.
"He could have very easily died."
She said Mr Mintz was shot in the back, abdomen, left hand and once in each leg but is expected to recover.
