On 4th March 2013, Northampton's Under-13 Academy left-sided striker Glenn Middleton joined Norwich for an undisclosed fee after he had been scouted four times by the Canaries.. “We could not stand in his way when he had the chance to join a Premier League Academy,” said Northampton's Head of Youth Trevor Gould. Middleton has been capped by Scotland at Under-16 and Under-18 level

Glenn signed a long term contract with Norwich in April 2015. He had benefitted from coaching by Darren Huckerby.

He was one of ten yougnsters to join Norwich's Under-18s as first year scholars on 1st July 2016.

Glenn signed a professional contract with Norwich on 1st January 2017.

Due to an injury crisis, Middleston was on the bench for Norwich's FA Cup Third Round replay defeat at Southamptonn on 18th January 2017.

In August 2017, he was called up to the Scotland Under-19 squad for an international tournament to be held in Serbia.

On 31st January 2018, he was surprisingly sold to Rangers. He signed an 18-month deal at Ibrox, with a sell-on clause agreed relating to his future development. His reason for leaving Norwich was a lack of first team opportunities but he will be part of Rangers' academy. He knows Rangers manager Graeme Murty from Murty's time at Carrow Road.

He made his first team debut for Rangers in July 2018 in the Europa League playing the final 10 minutes of a 2-0 home win over Macedonian side Shkupi and then coming on in the 64th minute of the second leg - a 0-0 draw which sealed progress for Steven Gerrard’s side. Rangers Assistant manager and former Leeds and Liverpool midfielder Gary McAllister said: “He has done very well and he just needs to build on what he is doing, continue his progress and listen to the advice of the coaches and the manager. We have got a player there.”

In August 2019, Middleton was linked with loan moves to both Dutch side Den Haag and Scottish side Hibs. This was finalised on 13th August 2019. He scored five goals and contributed five assists for parent club Rangers in 2018/19 and will be with Hibs for the rest of the 2019/20 campaign. The loan was cut short on 27th December 2019 as he hadn't played since the end of October 2019 due to other Hibs players returning from injury. During January 2020 he was linked with loan moves to Ross County and St Mirren.

At the end of the January 2020 transfer window, he was linked with a loan to Shrewsbury but instead completed a loan to Bradford City on 1st February 2020. "Glenn is a great capture for us and we are delighted to have finally been able to announce his signing. He is a very exciting player, who will bring flair and dynamism to our midfield." said manager Gary Bowyer.

Having made just one appearance in 2020/21, Rangers loaned him to St Johnstone on 25th January 2021 for the remainder of the season. He helped St Johnstone win the Scottish FA Cup defeating Hibernian 1-0 on 22nd May 2021 with Middleton playing 83 minutes but missed a penalty.

Middleton was loaned back to St Johnstone on 6th August 2021 for the entire 2021/22 season. He made 35 appearances, scoring twice.

In July 2022 he joined Dundee United for an undisclosed transfer fee, signing a three year contract. In April 2024 he helped Dundee United secure the Scottish Championship title, with a 0-0 draw at Airdrieonians.