For a simple graph $G$, the generalized adjacency matrix $A_α(G)$ is defined as $A_α(G)=αD(G)+(1-α)A(G), α\in [0,1]$, where $A(G)$ is the adjacency matrix and $D(G)$ is the diagonal matrix of vertex degrees of $G$. This matrix generalises the spectral theories of the adjacency matrix and the signless Laplacian matrix of $G$. In this paper, we find $ A_α $-spectrum of the joined union of graphs in terms of spectrum of adjacency matrices of its components and the zeros of the characteristic polynomials of an auxiliary matrix determined by the joined union. We determine the $ A_α$-spectrum of join of two regular graphs, the join of a regular graph with the union of two regular graphs of distinct degrees. As an applications, we investigate the $ A_α $-spectrum of certain power graphs of finite groups.