The route starts in the hamlet of Hartsdale and ends in the community of Grasslands at the boundary between the towns of Greenburgh and Mount Pleasant.

NY 100A winds northwest through the hills of Greenburgh as West Hartsdale Avenue, passing multiple residences on its way up.

Just after Woods End Lane, the route turns northward through Greenburgh, passing Ridge Road County Park and the Metropolis Country Club before reaching a junction with NY 100B (Dobbs Ferry Road).

NY 100A continues northeast through Elmsford, running along the southeastern edge of the Knollwood Country Club and passing an entrance to Westchester Community College.

After passing the entrance, the route winds northward through Greenburgh, passing multiple residences before reaching a junction with NY 100 (Grasslands Road) and NY 100C (Grasslands Road) just east of the Sprain Brook Parkway.