Leen studied mathematics (and electronics) at the University of Amsterdam (UvA). He helped set up the first Computer Science curriculum at
that university at the beginning of the 1980's. He got his PhD from the UvA in Mathematics and Sciences (in particular computer science) in
September 1986 and was subsequenty appointed assistant professor and promoted to associate professor in 2001, which is his current position.
Leen was a member of the Institute for Logic Language and Computation (ILLC) since it was founded in 1991.
He was director of the BSc computer science, and scientific director (ai) of the ILLC. He is
currently still a member of the ILLC, where he is now the programme leader of the Logic and Computation project (LoCo).
Leen has (co)-authored some 50 papers in computer science on various subjects, like datastructures, algorithms, complexity theory, Kolmogorov complexity,
Machine Learning, and related subjects, and a book on semi-feasible computation.
Leen has taught courses on Programming, Programming Languages, Automata Theory, Computability, Complexity Theory, Kolmogorov Complexity, Complexity of Language Systems,
Learning, Relational Databases, NoSQL Databases, and related subjects.