[1] She competed for Great Britain in the 1964 Summer Olympics held in Tokyo, Japan in the 4 x 100 metres relay, where she won the bronze medal with her team-mates Mary Rand, Daphne Arden and Dorothy Hyman.

Simpson competed for England in the 1966 Commonwealth Games held in Kingston, Jamaica, in the 4 x 110 yards relay, where she won the silver medal with her team-mates Maureen Tranter, Daphne Slater and Jill Hall.

She also was a member of the Great Britain team that won a gold medal in the 4 x 400 m relay at the 1969 European Championships in Athletics in Athens, Greece, setting a world record time of 3:30.8 minutes.

Running the third leg, Janet ran the joint fastest time (52.1) of the British quartet and made up 15 metres on the leader, Eliane Jacq of France.

She retired from athletics in 1969 but made a comeback to compete in the 1972 Summer Olympics in Munich, helping the Great Britain relay squad finish fifth in the final of the 4 x 400 metres in British record time (3:28.75).