NY 100 was first designated in the 1930 renumbering of state highways in New York, when many of the state roads in Westchester County were first publicly posted with route numbers. Originally, it went north from the Croton Reservoir to US 6 (now NY 6N) in Mahopac Falls on what is now NY 118, Baldwin Place Road, and Myrtle Avenue. Also assigned at this time was NY 118, which began adjacent to the Croton Reservoir at NY 100 and proceeded northeast along the Croton Turnpike to NY 22 in Croton Falls via modern NY 100 and US 202. The alignments of NY 100 and NY 118 north of the Croton Reservoir were swapped , placing NY 118 on the more westerly alignment and NY 100 on the Croton Turnpike between the reservoir and Croton Falls.