The 1800 Roadster, model number 18TR, was designed in the closing days of World War II. Triumph had been bought by the Standard Motor Company in 1944, and the managing director of Standard, Sir John Black, wanted a sports car to take on Jaguar, which had used Standard engines in the pre-war period. Frank Callaby was selected to style the new car. After getting Black's approval for the general shape, Callaby worked with Arthur Ballard to design the details of the body. Design of the rolling chassis was by Ray Turner. Walter Belgrove, who had styled the pre-war Triumphs and was employed as Chief Body Engineer, had no part in the design.]

Early post-war steel shortages meant that most of the body was built from aluminium over an ash frame, using rubber press tools that had been used making panels for the largely wooden bodied Mosquito bomber that had been built by Standard during the war. Only the front wings were made of steel. The frame was hand welded up from steel tube. The engine was a version of Standard's 1.5-litre, four-cylinder side-valve design that had been converted to overhead valves by Harry Weslake and built by Standard exclusively for SS-Jaguar before World War II. The Triumph version featured a downdraught Solex carburettor instead of the Jaguar's side-draught SU, and a 6.7:1 compression ratio instead of 7.6:1. A four-speed gearbox with synchromesh on the top three ratios was used.

The tubular steel chassis was a short-wheelbase version of the 1800 saloon, featuring transverse leaf sprung independent suspension at the front and a live axle with semi-elliptic springs at the rear. The rear track was wider than the front by 4 inches. Brakes were hydraulic.