Pittsburgh was, as far as I know, the last major American city to close its streetcar network. While there were some networks which survived (and two lines in Pittsburgh did) they are all weird exceptions which made abandonment and replacement with busses impractical. Pittsburgh, however, limped along until about 1975, although by that point the network had already been greatly diminished.
IIRC, the North American cities which didn't abandon their 1st gen streetcar networks were as follows:
Philadelphia: Remaining lines fed into a subway, although purely street-running lines survived well into the 1990's and one heritage line still operates
Boston: Fed into underground tunnel or fully separated ROW (Ashmont-Mattapan), large abandonments still occurred on feeder lines to subway
Cleveland: Segregated ROW interurban with downtown subway
Newark: Segregated ROW, downtown tunnel
San Fran: Tunnels under hills and central subway tunnel
New Orleans: Segregated ROW
Pittsburgh: Tunnels under mountains
Mexico City: Segregated ROW
Toronto: For some reason didn't abandon it despite having no tunnels or weird circumstances.