The shooting, around 5:15 a.m., was at a trailer park in Gentilly, a section of the city that flooded during Katrina. A few hours earlier, another man was shot and killed near the Guste public housing complex.
The latest violence brought the number of homicides in the city to at least 32 this year. Last year, New Orleans counted 161.
Police spokeswoman Sabrina Richardson said the security guard was shot in the face in a security booth at a gate leading into the site. She said two other security guards were on duty at the time, but one was at the park's back gate and another was in a bathroom. The other guards told investigators they did not hear the shot, Richardson said.
There had not been any major crime problems at the trailer site before Monday's shooting.
"It's always been safe. I've never at any time felt in harm's way," resident Kenny Randle said. "That doesn't change. All things happen for a reason. But this, man, this was just senseless."
Steven Nicholas, a chief in the police department, said the guard was about 50 years old. A motive for the shooting had not been established, although he said "robbery does not appear to be a motive."
Police shut down the exits to the trailer park and went door to door asking residents about the incident.
The Federal Emergency Management Agency had personnel on the scene Monday morning, but did not immediately have any details about the incident.
FEMA officials on Monday were also dealing with the evacuation of nearly 60 families from an FEMA trailer park in Hammond, about 45 miles northwest of New Orleans, that had been plagued by sewage leaks and power outages.
"They know how to put me out, but they don't know how to help me out. That's how I look at it," said one resident, Alsee Tobias. FEMA officials said Monday that all the displaced families had at least temporary new homes and they hoped to dismantle the site by the end of the week.