The eastern portion of NY 421 was built in the 1920s under the terms of a 1923 law authorizing the construction of a highway leading from modern NY 30 north to Warren Point. This road was completed in 1925.
A part of NY 421 on the eastern side was built in the 1920s as part of an agreement made in 1923 to create a route from NY 30 north to Warren Point. The project was completed by 1925, providing access to that area.