Born in Bucharest, Marin started playing football at the age of six with local Pro Luceafărul, and joined the academy of Viitorul Constanța seven years later.

[10] He made his first Belgian First Division A appearance two days later, coming on as an 80th-minute substitute in a 0–3 loss to Club Brugge at the Stade Maurice Dufrasne.

[12] On 20 September, he netted once and also provided an assist in a 4–0 victory over Heist for the Belgian Cup;[13] in December, he scored in league fixtures against Waasland-Beveren, Sint-Truiden and Kortrijk respectively.

[17] Standard played league winner Club Brugge in the Belgian Super Cup on 22 July 2018, with Marin being once again in the starting eleven, however his team lost the match 1–2.

[18] He recorded his first European appearance for Standard in the UEFA Champions League's third qualifying round 2–2 home draw with Dutch side Ajax, on 7 August.

[19] Marin then played all six matches in the group stage of the UEFA Europa League, as his team finished third behind Sevilla and FC Krasnodar.

[22] On 4 April 2019, it was announced that Ajax signed Marin on a five-year deal for a reported transfer fee of €12.5 million, with the player due to join the squad in the summer.

[33] Ten days later, he scored a goal in a 3–0 win over Ukraine in the opening group match, contributing to his country's first victory in the competition in 24 years.