The 32-story hotel on the Las Vegas Strip appeared to suffer serious damage to the roof and upper floors. An investigation into the cause of the three-alarm blaze and extent of loss was underway.
The fire broke out on the roof late Friday morning and forced the evacuation of the hotel as a huge crowd gathered to watch smoke and flames billowing from windows and the casino's facade.
Clark County Fire Chief Steven Smith told reporters it took firefighters more than an hour to contain the blaze.
Smith said the flames blackened the exterior of the building, which may have limited interior damage, but made the fire tougher to knock down. "Our firefighters had to hang out of the windows," Smith said. "It wasn't an easy fire at all."
It was not known how long the Monte Carlo would be closed. Guests were beingrebooked at other hotels owned by MGM Mirage.
Las Vegas was the scene of one of the worst hotel fires in the United States when the MGM Grand burned in 1980, killing 84 people and injuring 679.