Garmston, born in Chorley, Staffordshire, is a product of the West Bromwich Albion academy, making a number of appearances for the club's development squad. He was educated at The Cathedral School, Lichfield, and King Edward's School, Birmingham. He was spotted by West Bromwich Albion whilst playing for non-League side Sutton Coldfield Town. Garmston signed an initial one-year contract with West Brom. The club, again, opted to take up their option of a contract extension on Garmston to stay for another season. Garmston made his West Brom debut in the pre-season friendly against Port Vale on 5 August 2014. Garmston was given number thirty-three for the 2014–15 season. At the end of the 2014–15 season, Garmston was released by the club.
On 14 February 2013, Garmston joined Colchester United on a one-month youth loan, later extended to the end of the 2012–13 season. He made his Football League debut in Colchester's 3–0 away defeat to Sheffield United on 16 February. He made his 13th and final appearance for Colchester on the final day of the season, coming on as a substitute for George Porter in a 2–0 win at Carlisle United, helping the club avoid relegation.
After being told by the club's management that many youngsters to be loaned out last October, it announced on 16 January 2015, Garmston moved to Gillingham on a one-month loan. Garmston made his Gillingham debut on 17 January 2015, making his first start, in a 3–1 win over Coventry City. Garmston provided assist for Bradley Dack to help Gillingham beat Peterborough on 10 February 2015 and then scored in the next game, on 14 February 2015, in a 4–2 win over Milton Keynes Dons. Two days later, Garmston extended his loan spell with the "Gills" until 15 March 2015. After being released by West Bromwich Albion, Garmston re-joined Gillingham on a three-year contract. Garmston made his Gillingham debut since joining them in a pre-season friendly, in a 3–0 win over Folkestone Invicta on 7 July 2015. He was offered a new contract by Gillingham at the end of the 2017–18 season and released in January 2020.
Following his release from Gillingham, Garmston joined League Two side Grimsby Town, making five appearances before being released at the conclusion of the 2019–20 season.
