For how long has all of urban America been Democratic?

[Southern Senator North Carolina Yankee]

Urban American Democratic dominance is a product of the New Deal Era and was solidified by 1960 when the concept of the urban archipelago came into being.

NYC and Boston had been historically Democratic cities prior to this period, and Republican victories in 1920/1924 were the product of the Democratic collapse in the period leading up to those elections. Them flipping back wouldn't be very earth shattering. However, the rest of the cities on the other hand marked a huge shift and this was largely driven by suburban white flight, which removed the major Republican base, the transition of African-Americans to being strongly Democratic combined with the growth in their numbers during the great migration and lastly the shift in Republican policy focus away from one of industrialization, to one of opposing big gov't.

If you think about it, the Federalists, the Whigs and the Republicans thereafter, while involving larger segments of the rural north in each, had largely been urban and cosmopolitan oriented parties. Their base in each was middle/upper middle class and financial/mercantile elite. As voting expanded, they used a combination of machine politics, economic protectionism and internal improvements to maintain political power in a number of cities even with substantial demographic changes from immigration.

This fell apart once protectionism was removed from the table at the beginning of the New Deal Era. Republicans had already shifted away from internal improvements towards a more limited gov't orientation domestically, this weakened Republican power in the cities right as the Depression was beginning. Furthermore, the Smith candidacy had also shifted Republicans towards a more rural focus away from the cities at the expense of the urban political machines, just as Democrats were gaining ground in them.

By 1960, this process had fully developed, with Republicans dominating the suburbs where their base had largely moved to, but with Democrats increasingly dominating the cities.

Again, there were cities that were Democratic before this period, and there were cities that were Republican after this period (some of them in the Midwest and especially the Sunbelt, where this evolution of white flight and such was a few steps behind the rest of the county and the economic interests were different being more focused on the military and white collar jobs as opposed to big industry), but on the whole urban America has generally leaned Democratic for about 60 to 80 years.