Mixing of finite time-homogeneous Markov chains is well understood nowadays, with a rich set of techniques to estimate their mixing time. In this paper, we study the mixing time of random walks in dynamic random environments. To that end, we propose a concept of mixing time for time-inhomogeneous Markov chains. We then develop techniques to estimate this mixing time by extending the evolving set method of Morris and Peres (2003). We apply these techniques to study a random walk on a dynamic Erdős-Rényi graph, proving that the mixing time is $O(\log(n))$ when the graph is well above the connectivity threshold. We also give an almost matching lower bound.