Before the 1932 renumbering
In the 1920s, State Highway 123 was an east-west route from Cornwall to Canton:
- today's Route 4, from US 7 to Torrington
- former Route 4 (mostly today's US 202), from Torrington to Canton via Collinsville
At that time, present-day Route 123 north of New Canaan was called SH 184, and south of there, SH 302.
Norwalk Road (today's Route 123) was the first major road in New Canaan, connecting the town to Norwalk Harbor. In 1925, Norwalk officials submitted a bill to add a state highway along this road: from Nash's Corner in Norwalk via New Canaan Center and Selleck's Corners (Route 124 at West Road) in New Canaan to New York. This route is fairly close to the Route 29 shown on the 1932 highway map.
In 1931, New Canaan residents were asked to vote on the alignment of a new state highway leading from the town center to New York State. The route from Norwalk (along New Canaan and Norwalk roads) had already been established. The alternatives were:
- present Smith Ridge Road (today's Route 123, leading to Ridgefield and Danbury via New York)
- Oenoke Avenue (today's Route 124
- A new road, splitting into a Y at the north end, leading to Pinney's corner and Smith Ridge Road.
The Smith Ridge route narrowly beat out the new route, and soundly beat the Route 124 option.
Modern Route 123
Today's Route 123 was commissioned in 1932. Its original alignment, from south to north:
- Riverside Avenue, from West Avenue to New Canaan Avenue. The portion of Riverside Avenue south of Belden Avenue was later displaced by US 7 construction and no longer exists. The remainder, from Belden Avenue (US 1) to New Canaan Avenue, is now SR 809.
- New Canaan Avenue (today's Route 123) northwest into New Canaan
- Old Norwalk Road, Main Street and Forest Street in New Canaan. The New Norwalk Road section opened around 1935.
- Today's Route 123 into New York State
The 1932 official highway map contradicts the state's route change spreadsheet and other sources: there, from New Canaan center south, Route 123 goes to Darien while Route 29 (today's Route 124) goes to New Canaan. By 1934, these routes are in their usual locations. On the 1932 map, Route 104 and Route 137 are also swapped in Stamford.
Around 1935, the New Norwalk Road segment opened in New Canaan.
In 1970, Route 123 was moved from Riverside Avenue to Main Street in downtown Norwalk. This was one of several road changes resulting from US 7 freeway construction; that route opened in 1971. The leftover portion of Route 123 became SR 809.
In the 1980s Route 123 was for a time known as Beer Can Boulevard, because of a three-year differential between drinking ages in Connecticut (21) and nearby New York state (18). "Budd" writes: "...teenagers would run across the state line to drink...on returning it was littered with beer cans, wrecked cars and death, earning it its name."