Grontis began his career at the infrastructure segments of AEK Athens, signing a professional contract with the club in 2012.
[3] On 20 January 2016, Grontis moved to fellow Super League side Atromitos, signing a 2,5-year contract.
[5] His excellent performances helped his team avoid relegation, as Iraklis finished the season with 4 wins in its last 5 games.
He was however immediately loaned out to Football League Trikala,[6] and left AEL in the summer of 2018, signing a contract with second-tier club Aittitos Spata.
[7] After the first half of the 2018–19 season, Grontis moved to another second-tier club Ergotelis on a free transfer in January 2019.