In the 1920s, State Highway 123 followed former Route 4 between Cornwall and Canton; this is now Route 4 between US 7 and Route 8; and US 202 from there to US 44. 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.

The modern Route 123 was commissioned in 1932. Its original alignment, from south to north:

• Riverside Ave. (today's SR 809), from West Ave (US 1) to New Canaan Ave. (Route 123). The exact starting point was erased by US 7 freeway construction nearby.
• today's Route 123 to the New York state line

However, the 1932 official highway map has Route 123 following today's Route 124 south of New Canaan, but its own modern route north of there. By 1934, these highways were swapped in the south to the modern-day 123 and 124 alignments. Curiously, the same thing happened to Routes 104 and 137 in neighboring Stamford.

In 1970, Route 123 was moved from Riverside Ave. to Main St. in downtown Norwalk.

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."

• Chalder, Glenn. "Conditions and Trends: History of New Canaan."
• Connecticut Department of Transportation. "Route change table.xls". Microsoft Excel spreadsheet file, received April 4, 2008.

• Route 123 Ends (Photos; Doug Kerr at state-ends.com)