Mageean had won silver medals at the World Youth (2009) and Junior (2010) Championships. Her first senior international competition saw her finish 10th in the 1,500m at the 2010 Commonwealth Games in Delhi, representing Northern Ireland.
She competed in the 1500 metres at the 2016 European Athletics Championships, winning a bronze medal.
In 2016 Mageean became Irish indoor record holder for 1,500m and the mile. She qualified to represent Ireland at the 2016 Summer Olympics in Rio.
On 3 March 2019, she won a bronze medal in the 1500 metres at the 2019 European Athletics Indoor Championships.