Jallow was born in Munich, Germany as the first child of a German mother and a Gambian father.

[1] Growing up, Jallow looked up to Kobe Bryant but expressed admiration for Giannis Antetokounmpo later in his career.

[1] On July 14, 2015, Jallow signed his first professional contract with FC Bayern Munich of the Basketball Bundesliga.

[4][5] Team manager Marko Pešić said, "Karim is one of our biggest talents; he still hasn't stopped developing.

[7] In July 2019, Jallow signed a three-year contract with Basketball Löwen Braunschweig.