[10] Muir made her international debut at the 2011 European Cross Country Championships,[11] when she was part of the Great Britain junior women's team that won gold.

[14] She went on to compete at the 2013 World Championships in Athletics in Moscow, representing Great Britain in the 800 metres; she reached the semi-finals with a personal best time of 2:00.83.

[20][21] On 22 July, Muir broke Kelly Holmes' British record for the 1500 metres with a time of 3:57.49 to win the Diamond League event in London's Olympic Park.

[22] Less than two weeks later, on 27 August, Muir improved her own UK record by more than two seconds with a world-leading time of 3:55.22 to win the event at the Diamond League meet in Paris.

[23] A few days later, she became only the third British woman to win a Diamond Trophy as she won the 1500 m title with a second-place finish in Zürich, with the third fastest ever mark by a Briton.

On 4 January, racing the 5000 metres for the second time ever, Muir broke 25-year-old British indoor record held by her fellow Scot Liz McColgan, clocking 14:49.12 in Glasgow.

[26] Exactly a month later, she set a European indoor 3000 metres record in Karlsruhe in a time of 8:26.41, beating Russian Liliya Shobukhova's mark by 1.45 seconds and reigning Olympic 5000 m silver medallist Hellen Obiri.

Muir produced one of the most impressive wins of her career, beating three of the four fastest women in the world that year (Shelby Houlihan, Hassan and Gudaf Tsegay).

[40] In March, she became the first athlete in history to achieve the 'double-double' at a European Indoors as she defended both her 1500 m and 3000 m titles at Glasgow 2019, improving her own championship record at the longer distance.

[20] On 9 February 2021, she started her season well in Liévin, France, becoming the first British woman to break the four-minute barrier in the indoor 1500 m, and taking the record back from her Scottish training partner Jemma Reekie.

At the end of June, she lost to both Keely Hodgkinson and Reekie in the 800 m at the British Championships in 2:00.24 to set a personal best of 1:56.73 in July, when winning the Monaco Diamond League.

[20] At the delayed 2020 Tokyo Olympics in August 2021, Muir won the silver medal in the 1500 metres in a time of three minutes 54.50 seconds, improving her own British record.

[47] About two weeks later, Muir completed in just a 24-hour span the 800 m/1500 m double at the XXII Commonwealth Games in Birmingham, earning bronze in a photo-finish in the 800 m (0.01 s ahead of Natoya Goule and behind Mary Moraa and Hodgkinson) and, in the absence of Kipyegon, winning decisively gold for the 1500 m.[48][49] The 29-year-old continued her fine season successfully defending her 1500 m European title just 12 days after her Commonwealth gold.

[55][56] In January 2024, Muir was awarded a retrospective European Indoor bronze medal after Russian athlete Yelena Korobkina was found guilty of doping offences.

[58] After winning the 1500 metres silver medal at the 2024 British Athletics Championships, Muir was subsequently named in the Great Britain team for the 2024 Summer Olympics.

[59] On 7 July 2024, Muir set a new British 1,500 metres record as she came third in the Diamond League meeting in Paris with a time of three minutes 53.79 seconds.