[1] Glasspool made his ATP Tour debut (in doubles) at the 2016 Wimbledon Championships having been given a wildcard to partner compatriot Dan Evans in the main draw.

[2] Partnering Harri Heliövaara, Glasspool won his first ATP Tour title at the 2021 Open 13 against Sander Arends and David Pel of the Netherlands in March.

In May 2022, on their debut at the Italian Open, Glasspool and Heliövaara reached their first quarterfinal at a Masters 1000 level as alternates, defeating top seeds, world No.

[17][18] Partnering with fellow Briton Julian Cash, he won the Japan Open, defeating Ariel Behar and Robert Galloway in the final.

[19][20] At the Paris Masters where he partnered for the first time with Adam Pavlásek, the unseeded alternate pair reached the final with wins over fifth seeded Italian duo Simone Bolelli and Andrea Vavassori, local favorites Sadio Doumbia and Fabien Reboul, and US Open champions and fourth seeded Australian duo Max Purcell and Jordan Thompson.

[23][24] Reuniting with Julian Cash, Glasspool won the title at the Brisbane International as defending champion, defeating Jiří Lehečka and Jakub Menšík in the final.

[25][26] Glasspool and Cash defeated Ariel Behar and Robert Galloway in a deciding set tiebreak to reach the quarterfinals at the Australian Open,[27] where they lost to fourth seeds Kevin Krawietz and Tim Pütz.