Short answer is yes, casinos in Las Vegas were segregated.
Now the details. Luckily I just discussed the first half of this in my “History of Casinos” class, so it is fresh in my mind. When Nevada re-legalized commercial gambling in 1931, the Las Vegas City Commission specified that it would accept applications “a person of the Ethiopian Race for the conduct of a game or games in a place catering exclusively to persons of the same race only” (Las Vegas City Commission. Minute Book, Volume 3. 157.) Later that month, the Commission denied an application by William Jones and William Brown to operate a gambling hall at 217 Stewart Street (across the street from today’s Mob Museum), but in November it approved the duo’s application to operate a poker table and a dice table there, provided they “cater to the Ethiopian race only.”
Clarence Ray, who was one of the operators of the club, says that white patrons were present, but it appears that black gamblers were not welcome in the major clubs on Fremont Street, though Ray himself (who was black) worked in a few of them in the 1930s.
By the 1940s, black Las Vegans were restricted (de facto, not de jure) to living on the Westside, the site of the failed McWilliams townsite. Jackson Street, the main thoroughfare on the Westside, soon hosted a number of clubs open to all races.
On the Las Vegas Strip and Downtown, segregation was the rule. African-Americans were not accepted as patrons. In this letter to the New York Times, opera singer Robert Merrill describes Las Vegas in the mid-1950s, when he and Louis Armstrong split the bill at the Sands. They tried to “do the town” after a show, but Armstrong was denied entrance. Most black performers were allowed onstage but did not mix with guests after their shows. See, for example, Lena Horne’s cameo in Meet Me in Las Vegas, where she sings from the stage but is not seen in the casino.
The Moulin Rouge casino, which was far off the Strip at Bonanza Road on the Westside, was integrated, but it was open as a full-scale casino only for a few months in 1955 before it closed, likely due to poor financial controls (each of the new casinos that opened that year closed or changed ownership, so it was not unique in struggling).
By 1958, Sammy Davis, Jr. was apparently “allowed” to socialize in the Sands casino (I caught a mention of him walking the casino with manager Carl Cohen in a gossip column that year), but usually unwound after shows with friends in the second-floor Emerald Room. But most non-celebrity African-Americans would not be allowed through the door of any casino on the Strip or Downtown.
That changed in March 1960. The Las Vegas NAACP threatened to picket the Strip casinos if they did not integrate. As the city was pivoting to appeal to convention guests, the thought of racial strife and picket lines worried casino operators. If the hotels’ black employees and entertainers refused to cross the picket lines, there could be real trouble.
So, on March 26, 1960, representatives of the casinos, the NAACP, and local political leaders announced that the city’s casinos would desegregate. So after 1960, black guests were welcomed (the Sands soon started hosting a fashion show affiliated with Ebony magazine). Before that, not so much.
Interesting artifact of Las Vegas segregation. In the original Ocean’s Eleven, filmed in early 1960, Sammy Davis Jr.’s character is a garbage truck driver who is instrumental in ferrying the cash away from the casinos after they are robbed. It would have been understood at the time that he, unlike the other ten in Danny Ocean’s group, would not have been allowed in the casino. And, in the final scene, as he and the others are walking outside the Sands (not in it), he trails after the rest of the group, though he’s singing the final theme. A metaphor, perhaps, for the just-about-to-end segregation of Las Vegas casinos.