Haig was born in Marylebone, London in 1905, a member of the Scottish whisky-distilling Haig family and grand-niece of Field Marshal Douglas Haig. Her early years were spent at the family estate in Ramornie, Fife. After her parents divorced in the early 1920s she moved to Sussex with her mother, and her passion for machines began early: she bought her first motorised vehicle, a surplus 2¾hp Douglas motorcycle, at the age of 14, and her first car, an Austin 7 Sports, at 16. She attended the very first British Grand Prix in 1926 and was introduced to Brooklands through a lap in her then-boyfriend Denis Sprague's Talbot 8. She married Sprague in January 1928, but the marriage did not last, and she spent time in Africa before returning to England.
Haig entered her first competition, the Junior Racing Drivers Club Speed Hill Climb at Chalfont St Peter, in 1934, driving a Singer Nine Le Mans she had recently acquired. That same year she and companion Joyce Lambert entered the Rallye Paris–Saint-Raphaël Féminin, completing the event and impressing Singer sufficiently to earn factory support. Her first circuit appearance came in 1935 at the JCC High Speed Trials, driving an Aston Martin.
The peak of her pre-war career came in 1936 when she won a gold medal in the Olympic Rally, held in conjunction with the Berlin Summer Olympics. Driving a six-cylinder Singer Le Mans 1500 with co-driver Joyce Lambert, Haig took the overall victory in a competition that stood as the sole Olympic automobile race of the modern era.
She also contested the Rallye Paris–Saint-Raphaël Féminin multiple times, winning outright in 1938 and 1939 in an MG PB. At the 1946 Rallye des Alpes Françaises she finished second overall with a class win and the Coupe des Dames, co-driven by Enid Riddell in a 1937 AC 16/80. In 1949 she returned to the Alpes with co-driver Barbara Marshall in an MG TC, finishing seventh overall and winning both her class and the Coupe des Dames again. At the 1951 24 Hours of Le Mans she finished fifteenth and third in class, co-driving a Luigi Chinetti Ferrari 166MM Coupé with Yvonne Simon. The Monte Carlo Rally also featured in her record: in 1949 she finished second in the Ladies Class in a Morris Minor alongside co-drivers Elsie Wisdom and Barbara Marshall.
In the post-war years Haig focused increasingly on club events and hillclimbs, driving single-seaters including a Cooper 1000 in 1950 and a Cooper 500 in 1951. She won the National Ladies Hillclimb Championship in both 1960 and 1961 driving a Coventry-Climax-powered Lotus Seven, and held the Ladies' hillclimb record at Prescott for six years.
In 1966, Haig and photographer Guy Griffiths founded the Historic Sports Car Club (HSCC), which became one of the leading organisations for historic motorsport in Britain. She was also a charter member of the Frazer Nash Car Club and the Porsche Club of Great Britain, and worked as a journalist and race reporter for Motor Sport magazine beginning in the late 1940s.
Over her lifetime Haig owned more than 60 cars, ranging from everyday road vehicles to purpose-built racers representing marques including Bugatti, Jaguar, Lotus, Aston Martin, Cooper, Frazer Nash, and AC Cars. Among the most notable was the first AC Ace ever built (chassis AE 01), an aluminium-bodied pre-production Austin-Healey 100 that was the first right-hand-drive example, and a Jaguar XKSS converted from a D-Type.
She died early in 1987. Her memory is preserved through the Triple-M Register's Betty Haig Cup for best racing performance of the year, the Betty Haig Memorial Trophy for the fastest time by a lady competitor in a racing car at Prescott, and the AC Owners' Club's Betty Haig Trophy for the fastest lady member on handicap at Goodwood.