Hardcastle finished second in the 800-metre freestyle at the British national championships in 1982, aged 13. She was selected for the England team at the 1982 Commonwealth Games in Brisbane,^[2] where she reached the final of the 800-metre freestyle.^[3] In 1983 she won the bronze medal in the 800-metre freestyle at the European Championships, aged 14.^[4]

At the 1984 Summer Olympics in Los Angeles, she became the youngest ever British woman to win an Olympic medal^[5] when she won silver in the 400-metre freestyle, aged 15 years, 3 months and 22 days.^[6] She also won bronze in the 800-metre freestyle, and finished ninth the 400-metre individual medley.^[7]

Hardcastle won the silver medal in the 800-metre freestyle and finished sixth in the 400-metre freestyle at the 1985 European Championships.^[8] At the 1986 Commonwealth Games, she won gold medals in both the 400- and 800-metre freestyle. Her time of 8:24.77 in the 800-metre freestyle was the second-fastest ever recorded, 0.15 seconds outside the world record held by Tracey Wickham, and a new European record.^[9] Her time in the 400-metre freestyle was a Commonwealth Games record.^[10] She also won bronze in the 400-metre individual medley,^[11] and was a member of the team that won silver in the 4×200-metre freestyle relay.^[12] She won the bronze medal in the 400-metre freestyle at the 1986 World Championships.^[5]

Hardcastle initially retired from competitive swimming in 1986 at the age of 17; in a 2012 interview she said that after years of training she had wanted to have a normal teenage life.^[7] She participated in Prince Edward's charity television special The Grand Knockout Tournament in 1987. After a spell working as a secretary at Ford,^[13] she decided to return to the sport in 1992^[14] and resumed competing in 1993.^[15] Hardcastle was a member of the relay teams that won bronze in the 4×200-metre freestyle at the 1993 European Championships and silver at the 1994 Commonwealth Games,^[7] where she also won an individual bronze medal in the 400-metre freestyle.^[16] In 1995 she won the 800 m freestyle gold medal^[17] and bronze in the 400 m freestyle at the World Short Course Championships.^[18] At the 1996 Olympics in Atlanta she reached the final of the 800-metre freestyle, finishing eighth.^[19]

At the ASA National British Championships she is a five times winner of the 400 metres freestyle in (1984, 1986, 1993-1995) and four times winner of the 800 metres freestyle (1984, 1986, 1993 and 1995), in addition to being the champion over 400 metres medley in 1983 and 1986.^{[20][21][22][23][24]}

Hardcastle retired permanently from competitive swimming after the 1996 Olympics, later moving to New Zealand to work as a swimming coach.^[7]

Hardcastle was born in Chelmsford, Essex^[1] and attended Shoebury High School.^[7] Her mother, Ann Hardcastle, is a former swimming coach who taught several British elite swimmers including Mark Foster.^[25] Hardcastle married Lee Thomas in 1995;^[15] the couple have four children.^[7] Her daughter Eve Thomas^[26] is a current international swimmer representing New Zealand.

• List of Olympic medalists in swimming (women)
• List of World Aquatics Championships medalists in swimming (women)