Commissioned in 1944, Route 21 was originally a 3.78-mile connector from Route 12 to US 44. Part of this route, Converse Road, had already been added to the state highway system in 1936 as SR 856.

In 1955, Route 21 was extended another 1.89 miles north to Route 193.

In 1963, however, it appears the state wanted to extend Route 193 southward and absorb Route 21. The ConnDOT spreadsheet shows this happening that year, and no record of it being rescinded; but a look at a highway log or map shows that Route 21 survived that proposal.

Extend Route 21 southward along Route 12, SR 607, SR 664 and Route 49 to Stonington.

• Connecticut Department of Transportation. "Route change table.xls". Microsoft Excel spreadsheet file, received April 4, 2008.
• "Putnam Road Stretch Is Accepted by State." Hartford Courant, Dec. 22, 1955.

• CT 21 photos (Doug Kerr)