Duff was born in Jiujiang, China, to Canadian parents from Hamilton, Ontario,[1] who had established a commercial outpost in nearby Guling.

Gravely wounded at the Third Battle of Ypres, Duff was sent to a hospital in England where he met and married his nurse.

In 1920, Duff began his racing career at Brooklands, a 2.6 mile long concrete track with concave banking.

By August 1920, he was lapping in the same range as Henry Segrave, one of the great Brooklands and Grand Prix drivers of the 1920s.

Harry Ricardo made a set of aluminum pistons and raised the engine’s compression ratio.

On August 28, he took a stock 3-litre model to the track where he made an attempt on the "Double Twelve" record (24 Hour runs were not allowed at Brooklands due to the noise).

The car broke before he could achieve that goal but, in the process, Duff set new Class E world records for 1, 2, and 3 hours, 100 and 200 miles, and 100, 200, 300, and 400 km.

Duff returned to Brooklands on September 27–28, driving both 12 hour shifts singlehanded to take the Double 12 [5] at an average of 86.52 mph, for a total distance of 2,082 miles (3,351 km).

Unable to stop the old car at the end of the last lap, Duff shot over the top of the banking, crashing through trees and a telegraph pole outside the circuit before finally coming to rest.

[8] Rough track conditions took their toll as a flying stone holed the fuel tank, forcing Duff to run back to the pits.

His car was prepared alongside the works entry using ideas Duff had come up with after the 1923 race.

Partnered by Clement, in a race run in intense heat, Duff won handily, giving Bentley its first victory at Le Mans.

On September 9–10, 1925, Duff went to the high-banked Montlhéry track, near Paris, for an attempt at the absolute 24 Hour Record.

He had a special single-seater Weymann body on his Bentley and works driver Dudley Benjafield as his co-driver.

Driving on a damp track in heavy mist, they covered 2,280 miles in 24 hours, averaging 95.02 mph.

"John Duff of Indianapolis, Ind., wrecked his machine and suffered a broken collar bone when his car crashed through the top rail and dropped clear of the track.

"[17] Duff suffered painful bone and muscle injuries, and a concussion that affected his vision.

Knowing that his competitiveness would be compromised, and having promised his wife that he would quit if he suffered another serious injury, Duff retired from racing.

When the Depression led to a decline in the demand for fencing instruction, Duff moved his family to China.

The Duff family’s settlement at Guling had prospered over the years and turned into a thriving resort.

When John Duff started racing in 1920, he had lived a significant part of his life in Canada.