- Southern Terminus:
- Central Park Avenue (NY 100) in Hartsdale
- Northern Terminus:
- Bradhurst Avenue (NY 100) and Grasslands Road (NY 100/NY 100C) in Valhalla
- 4.21 miles
NY 100A is a loop off of NY 100 that bypasses the section in the downtown area of White Plains. It begins at Central Park Avenue (NY 100) in Hartsdale and runs to Elmsford, where there are grade-separated interchanges with Tarrytown Road (NY 119) and the Cross Westchester Expressway (I-287). Just north of the expressway, it changes names from West Hartsdale Avenue to Knollwood Road. It continues north to Valhalla, where it ends at NY 100 and NY 100C. The entire length is a two-lane undivided road.