Health officials made the discovery following the death of a guest. Three guests have been diagnosed in the past year.
The resort's water was tested after the first two cases were reported during the spring of last year, but no Legionella bacterium was detected, district officials said. Those guests recovered.
An MGM Resorts spokesperson, which owns the Luxor, said they immediately began treatment procedures.
The new cases come as the company is already facing a $337.5 million civil lawsuit from guests who said they were infected with Legionella at the Aria Resort & Casino, half-owned by MGM Resorts. Six cases of Legionnaires' disease were detected there in July. The district said those guests recovered after treatment.
Last year, the bacterium was found in a hot tub at the Playboy Mansion in Los Angeles. Dozens of people who attended a fundraiser there became sick.
The bacterium is commonly found in fresh water environments and causes respiratory illness. The Luxor hotel guest died of the form of pneumonia.
The Associated Press contributed to this story.