Fagerson signed his first professional contract in October 2017. The back-row made 18 appearances in his first full season as a professional, winning the Warriors' Most Improved Player of the Season award. In addition, Fagerson was awarded the Sir Willie Purves Quaich by Friends of Scottish Rugby, in recognition of the season's outstanding young Scottish player.
A physical back-rower, Fagerson represented Scotland at U16 and U18 level before representing Scotland in the 2017 World Rugby U20s Championship in Georgia, helping the team finish fifth.
His form in the 2017/18 season was rewarded with a maiden international call-up, which saw him win his first cap in a 30-29 defeat to the USA in Houston on the 2018 summer tour. He became just the second Scotland forward to be capped as a teenager in 67 years after Glasgow team-mate Jonny Gray, and just the third teenager to be capped by Scotland in the pro era, following in the footsteps of Gray and Stuart Hogg.