Hailing from Turku, on the south-west tip of Finland, Marcus is the grandson of former international footballer Rainer Forss, who made 14 appearances for Finland and was a member of their 1952 Olympic squad.

He moved to England to join the West Bromwich Albion academy at the age of 13, linking up with the Baggies as a scholar, before joining then-Championship outfit Brentford.

Initially turning out for the Bees’ B team, Marcus was named their Player of the Season during the 2017/18 season after bagging 21 goals.

Marcus then stepped up to the first-team squad the following season, scoring twice in nine appearances, and he was rewarded with a new four-year contract.

He joined AFC Wimbledon on loan in the early stages of the 2019/20 campaign, netting seven goals in his opening seven matches - including a hat-trick in a 4-1 win over Southend United.

Marcus followed up a first senior treble by scoring four goals in five appearances in October 2019, earning him the EFL Young Player of the Month award.

Another four goals in the month of December saw him nominated for League One Player of the Month, but Marcus suffered a torn hamstring in the January which ruled him out for the remainder of the season.

Despite registering just 19 appearances for the Dons, scoring 11 goals, Marcus was recognised as AFC Wimbledon’s Young Player of the Year and, after recovering from injury, he was immediately thrown into the first-team set-up at Brentford.

Scoring seven times in 13 games, including the decisive goal in their play-off semi-final win against Bournemouth, he was awarded with a new five-and-a-half-year contract as Brentford won promotion to the Premier League.

Making his full international debut in November 2020, Marcus broke the deadlock in a 2-0 friendly victory over World Cup holders France and was a member of Finland’s Euro 2020 squad.

After scoring five goals as the EFL Cup's leading goalscorer, he was named in the Team of the Tournament for 2021/22 and spent the back end of the season on loan at Hull City.

Marcus penned a four-year contract with Boro in July 2022, scoring his first goal for the club in only his second appearance, away at QPR.

In his first season at Boro, Marcus finished the season on double digits for goals. Marcus scored 10 goals in 38 Championship appearances.

The forward scored seven goals 17 appearances in the league in 2023/24.

His best spell came at the start of 2024, when he scored three goals in as many games, including a crucial strike at home to Sunderland.

In the 2023/24 campaign Marcus also made two more appearances for his national team in games against Slovenia and Kazakhstan.