Fractional differential equations (FDEs) constitute an essential field of mathematics which has been increasingly studied over recent decades, hence the importance of finding solutions for such systems. In this paper, we prove the existence and the uniqueness of the mild solution for a class of time-fractional evolution systems with an order of differentiation 1<α<2\documentclass[12pt]{minimal}
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\begin{document}$$1<\alpha <2$$\end{document}. We make use of different properties of the cosine and sine families as well as the Laplace transform (LT) to obtain a simplified expression of the mild solution, which will be given as an integral formula involving the Mainardi’s wright-type function. Moreover, many useful properties related to the operator appearing in the mild solution are given with proofs. Finally, we provide an illustrative example along with the expression for its mild solution.