Halsall was the youngest member of the Team England swim squad at the 2006 Commonwealth Games in Melbourne, where she won silver medals in the 4×100-metre freestyle and 4×100-metre medley relays.

Halsall won five medals at the 2010 European Championships in Budapest, 2 Golds, 2 Silvers and a Bronze and was a member of the 4x100-metre medley relay team that successfully defended their title for a second time.

At the 2010 Commonwealth Games in New Delhi, India, Halsall won a gold medal, with a national record of 26.19 seconds, in the 50-metre butterfly, beating the favoured Australians.

In the 100-metre freestyle Halsall was expected to win easily but a bout of illness (Delhi Belly) left her pale and weak and she had to be content with the bronze medal.

Having just missed out on a medal in the 50m butterfly by coming fourth, Halsall successfully gained bronze in the 50m freestyle posting a time of 24.30 seconds.

This ended a disappointing drought of medals for Great Britain just one year after the London Olympics, though Halsall was the only swimmer to finish in a medal-winning position.