I know most of Southern California had 50Hz until the late '30s, and some parts until after WWII.
The conversion of the Los Angeles area from 50 to 60Hz in the 1930s was discussed here some time ago. I can't locate the thread at the moment, but a few months ago we also had a fairly long discussion about localized variations. It was after I acquired a 1950s book on model railroading which suggested that some parts of the west coast were still on 50Hz and that parts of upstate New York used 25Hz.
And of course DC mains were still common in urban centers as late as the '50s.
As was the case in Britain. DC supplies were common in the older parts of the larger cities, i.e. those districts which were the first to have reticulated power.
