The Thousand Islands Parkway was constructed as a divided highway during the late 1930s, alongside the Thousand Islands Bridge, which opened in 1938. Originally known as the St. Lawrence River Road, the parkway was signed as part of Highway401 when the 400-series highway system was established in 1952, but was bypassed by the current Highway401 alignment to the north in 1968. The Highway2S designation returned between 1967 and 1970, after which jurisdiction over the parkway was transferred to the St. Lawrence Parks Commission. The northern carriageway of the parkway was never paved, and was only in use by vehicles between 1938 and 1951. Evidence of its former use can be seen today in the wide right-of-way; the unused westbound lanes now serve as a recreational trail and twin bridges span two locations along the parkway.