The roadways that makeup NY 118 through
Westchester and
Putnam counties was taken over and constructed between 1903 and 1907. The first two segments, unsigned State Highway 148 (SH 148), which was NY 118 between a point north of
NY 129 to Allan Avenue in
Yorktown Heights along with a segment along the
New Croton Reservoir and SH 149, which went from Allan Avenue all the way to
US 6 in
Baldwin Place, were both contracted for improvement on June 16, 1903. The contract on SH 148, long, was completed August 5, 1905 at the cost of $34,272.91 (equivalent to $ in ), half paid by the state. The contract on SH 149, long, cost $51,232.71 (1905 USD) and was completed on October 28, 1904. The other portion of NY 118, SH 405, made up part of
NY 100, was contracted on September 20, 1907, at the cost of $17,605 (equivalent to $ in ). This portion of NY 118 and NY 100 was completed and accepted into the state highway system on November 21, 1908. NY 118 was assigned as part of the
1930 renumbering of state highways in New York; however, it did not initially follow any of its modern alignment. Instead, NY 118 headed northeastward from its current southern terminus on modern NY 100 and
US 202 to a terminus at
NY 22 in
Croton Falls while all of what is now NY 118 was part of NY 100. At the time, NY 100 continued north along Baldwin Place Road and Myrtle Avenue to meet then-US 6 near
Mahopac Falls. In June 1934, the portion of NY 118 between
Somers and Croton Falls became
concurrent to US 202, a new interstate route extending from
Delaware to
Maine. The alignments of NY 100 and NY 118 north of New Croton Reservoir were flipped , placing NY 118 on its modern alignment. NY 118 initially continued north to
NY 6N over NY 100's original alignment; however, this extension was eliminated in the mid-1940s. With the flip, NY 118 terminated at a junction with NY 129 in Croton Lake, while the latter extended to a
traffic circle with NY 100 in the hamlet of
Pines Bridge. This alignment remained until , when the route was extended over the easternmost piece of NY 129 to end at the traffic circle with NY 100. The traffic circle in Pines Bridge was removed between 1988 and 1991 and replaced with a three-way intersection between the two routes. ==Major intersections==