I've never been able to find an actual date for the establishment of the Groom Lake base.
Some sources say it was built for the CIA in the late 1940s but the fact the site was used for bombing practice and artillery training in WW2 suggests differently.
Of course, it may be that the land was owned by the US military before or during WW2 without having an actual base built on it - just the essentials for training. At that time it was called "Nellis Auxiliary Field No.1".
In any case, the base (as it currently stands, with expansions at several times) became properly operational in itself in the mid-1950s when it was used for U2 testing, thanks to its extremely remote location.
Before U2 testing was set up, the airfield was apparently abandoned, overgrown and unusable - the lake bed itself was the primary attraction.