The aim of this thesis is to prove the existence and uniqueness of the mild solutions of semilinear initial value control problems in a suitable Banach spaces as well as their controllability. Some theorems regarding controllability, local and global existence as well as uniqueness of the mild solution in infinite dimensional spaces have been developed in suitable Banach space using the Schauder fixed point theorem and the semigroup theory (compact semigroup). By using the Banach contraction principle and the semigroup theory (analytic semigroup) in infinite dimensional spaces, have been discussed and developed in suitable Banach spaces the local existence and uniqueness of the mild solution to the semilinear initial value control problem. Some illustrations and practical scopes of the problems have been discussed and present.