Born Kathleen Coad Defries in Canada but raised in England, Petre was a star at the legendary Brooklands track. The exploits of this 4'10" lady caused a media sensation at the time. The abiding image of Kay is a tiny woman seated in a huge 10.5 litre V12 Delage. This was the car in which she battled for the Women's Outer Circuit Record at Brooklands with Gwenda Stewart. Kay gained the upper hand on 26th October 1934 with a 129.58mph lap, but in August 1935 Gwenda fought back with a faster lap. A determined Kay took her record back the same day with a 134.75mph pass but Gwenda, driving the Derby-Miller, had the last laugh three days later at 135.95 mph.
Although she is always associated with the Delage, Kay started racing in a Wolseley Hornet Special bought for her by her husband. She also raced an Invicta and a Bugatti in which she won a handicap race in 1935. However, she was most successful in a series of Rileys. She was ninth in the Mountain Grand Prix at Brooklands in a Riley 1.5 in 1934, against tough opposition. Her first visit to Le Mans was also that year. She and Dorothy Champney finished thirteenth, driving a Riley Ulster Imp. The Riley connection continued next year but Kay and Elsie "Bill" Wisdom failed to finish with a blown engine.
Between 1934 and 1936, Kay was a regular at all the big British races like the Brooklands 500 Miles and Double Twelve Hours, plus sports car races at Donington Park and Crystal Palace. She partnered some big names, such as Dudley Benjafield and Prince Bira of Siam. She also drove in rallies and was an accomplished hillclimb driver, claiming the Ladies' Record at Shelsley Walsh twice.
In 1937 Petre travelled to South Africa for the Grand Prix motor racing season with her Riley. Here she befriended the legendary Bernd Rosemeyer, who was racing for Auto Union. Competing against him and other top drivers of the day, she drove in three Grands Prix, scoring a sixth place in the Grosvenor GP but failing to finish the others. In September of 1937, she went to France to race a "Grasshopper" Austin in the Paris to Nice rally.
She was driving for the works Austin team at Brooklands when her career was ended by a terrible accident. During practice for the 500 Mile race, Reg Parnell misjudged an overtaking move, lost speed, slid down the banking and hit her Austin Seven from behind. She crashed badly and was seriously injured. She never raced competitively again.