This vintage map from 1867 shows Lower Westchester County 7 years before becoming The Bronx. It’s a lesser known fact that The Bronx was the first borough to become part of New York City.
In 1874, the lands west of the Bronx River were annexed to New York County aka NYC.
Before this moment, these lands were part of Westchester County and included the towns of Morrisania, Kingsbridge, and West Farms. Within these towns were the villages of Mott Haven, Port Morris, Melrose, Fordham, and many others.
Then in 1895, a full three years before Brooklyn, Queens, and Staten Island were annexed to create New York City as we know it today, the lands to the east of the Bronx River were annexed.
Read through this short article for more on this.