She started in athletics at school in Essex with her twin sister Iris. Aged 18, she finished second to Joyce Smith in the mile race at the WAAA Championships. She won the following year in a championship record time of 4:47.9, retained the national mile title in 1967 and won the newly introduced 1500 metres in 1968, 1970 and 1971. She won the national cross country championships in 1969, 1970, 1971, 1972 and 1974. In 1974 she also won a bronze medal in the IAAF World Cross Country Championships.
In 1969 she achieved a British record time of 4:15.9 in the 1500 m and she set a new record of 4:15.4 in 1970 and then 4:14.3 and 4:12.65 in 1971. In December 1968 she was the first British woman to break 10 minutes in the 3000 metres with a time of 9:59.6. She won the gold medal in the 1500 m  at the 1970 Commonwealth Games in Edinburgh. In a fiercely contested race, New Zealand's Sylvia Potts tripped and fell just one metre from the finish line, with Ridley carefully avoiding the falling athlete on her outside, to take the title.