Arguably, the greatest ever mathematical logician, Emil Leon Post, was among the main founders of Computer Science (Informatics/Informatique).
The fate was cruel to him, in more than one way, hence no wonder that he is vastly unappreciated.
Emil Post had significant contributions to algebra too. But let's concentrate on mathematical logic.
People don't appreciate the Emil Post's theorem about the elementary logic: tautologies = theorems. It may seem trivial but there are hardly any textbooks which include a complete(!!) proof. There is an objective reason why this theorem is not trivial. Indeed, a minor modification of the axioms of Boolean algebra leads to systems which are very hard to tell from actual Boolean algebras. On occasions, it takes intensive computer programs to decide the issue.
Emil Post had developed formalization independently of David Hilbert (there are trade-offs between the approaches by these two mathematicians).
Emil Post has proved the incompleteness theorem years before Kurt Gödel (again, there were trade-offs between the two).
Emil Post has developed the theory of algorithms independently of Alan Turing; occasionally, people talk about Post-Turing machines.