It ran for 2.63 miles (4.23 km) between an intersection with U.S. Route 4 in North Greenbush and a junction with NY 66 just inside the Troy city limits.

It headed eastward and northeastward on Winter Street through a residential area to an intersection with NY 136.

[1][4] NY 40 initially extended south of Troy to East Greenbush when it was assigned as part of the 1930 renumbering of state highways in New York.

As part of numerous changes by the New York State Department of Transportation (NYSDOT) in February–March 1973, a new alignment would have NY 40 run down Oakwood Avenue to Hoosick Street in Troy, ending there.

[2] On April 1, 1980, ownership and maintenance of NY 405 from US 4 through its junction with NY 136 to the Troy city line was transferred from the state of New York to Rensselaer County as part of a highway maintenance swap between the two levels of government.