On 1 September 2018, Mavrommatis made his first professional appearance for the team in the 2018–19 Superleague Greece game against Lamia.
[1] In January 2020, Theofanis Mavrommatis was shifted out on loan to Danish club SonderjyskE after a disappointing first half of the season, but he showed promise with reliable performance in Denmark before the coronavirus pandemic halted the Superliga.
Reports have suggested that Mavrommatis will be given another chance by Giorgos Donis, but the centre-back will only be used as a back-up option.
On 31 January 2021, following his release from Panathinaikos, Mavrommatis signed a contract with Atromitos, until the summer of 2022.
[2] On 28 July 2023, Mavrommatis joined Saudi Arabian club Al-Adalah.