The first "title fight" I could find was Mike Welia beating Eddie Hudson for the the Nevada State Welterweight Title at the Hotel Last Frontier Arena on 6/8/1946.

Archie Moore beat Nino Valdes for the Nevada World Heavyweight Title at Cashman Field on 5/2/1955.

The first non-Nevada title fight was Benny 'Kid' Paret beating Don Jordan for the World Welterweight Title at the LV Convention Center on 5/27/1960.

The first boxing association match was when Gene Fullmer beat Sugar Ray Robinson for the NBA World Middleweight Title at the LV Convention Center on 3/4/1961.

Is the last one the fight you were looking for?