This runs contrary to the New York State Department of Transportation's standard practice of inventorying an east-west route from west to east.

It proceeds eastward as Fulton Street in Farmingdale, a four-lane commercial arterial through the town, passing multiple apartment buildings.

At Main Street, a median starts developing in the center, dividing NY 109 as it crosses through a residential section of Farmingdale.

The four-lane arterial crosses through a large industrial section of Babylon, entering an intersection with County Route 3 (CR 3; North Wellwood Avenue).

Changing over to a commercial neighborhood of Babylon, NY 109 remains four lanes, crossing an intersection with CR 2 (Straight Path).