Route 198 was commissioned in 1932, and originally followed Bigelow Hollow Road, leading from the old Route 15 (later upgraded to I-84) in Union to the old Route 91 (now part of Route 171) in Kenyonville.

By 1942, the route (or at least the state-maintained part) had shrunk to 2.60 miles, from the Union/Eastford town line to state Route 91.

Between 1947 and 1949, its official definition was re-extended to Route 15, for a length of 8.99 miles. Between 1952 and 1954, it was extended along Buckley Highway to the new Route 15 freeway, for a length of 9.45 miles. This probably happened in concert with the deletion of Route 15A along the Buckley Highway.

In 1959, the state route 91 numbering was deleted, to avoid conflicts with the new Interstate 91. Phoenixville Road, Chaplin Road, and Eastford Road (from US 6 to today's Route 171) were given to Route 198, which now extended 21.35 miles, from US 6 to Route 15.

In 1963, Routes 198 and 171 (along with dozens of others) were re-arranged, and 198 was relocated along Black Pond Road and Eastford Road into Massachusetts.

In 1998, Route 198 officially shrunk 0.05 mile as the US 6 reconstruction moved its intersection that far north.

• Connecticut Department of Transportation. "Route change table.xls". Microsoft Excel spreadsheet file, received April 4, 2008.

• State route 91
• State route 171
• Route 198 Ends (Photos; Doug Kerr at state-ends.com)
• Route 198 - Google Maps