Born in Gävle, Sweden, to a Kenyan mother and Swedish father, Marcus Olsson started his career at Högaborgs BK. In 2008 he signed a contract with Halmstad,replacing Martin Fribrock as left winger as Fribrock departed for Esbjerg fB in the summer of 2008. Mainly featured as left winger, he also used as a forward in several games, during Josep Clotet Ruiz time at Halmstad and the lack of a natural left side defender, Olsson and Kujtim Bala took turns on playing on the position. As Halmstads BK was relegated from Allsvenskan in 2011, Marcus announced that he would not sign a new contract with the club and that he would start searching for a new club to play for.
Blackburn Rovers confirmed on 31 January that Olsson had signed a two-and-a-half year contract with the club, joining his twin brother at the club. Marcus Olsson made his debut for Blackburn Rovers in the 3-0 away defeat to Manchester City in the Premier League on 25th February 2012, playing on the left wing, in front of his twin Martin who played at left back, with both completing a full 90 minutes.
Marcus Olsson was called up to the annual January tour of the Swedish national team in 2012, facing Bahrain and Qatar U23.
His twin brother, Martin Olsson, also plays for Blackburn Rovers in the Football League Championship. Sister, Jessica Olsson is an associate director at an art gallery, and engaged to Dirk Nowitzki of the Dallas Mavericks.