The Kemeny's constant of a graph \( G \), denoted by \( \kappa(G) \), is defined as the expected time to travel from a fixed starting vertex to a random destination vertex (according to the stationary distribution). This constant is shown to be a novel resistance distance-based graph invariant, which indicates its crucial application in chemistry. In this paper, comparison result on Kemeny's constant of \( S,T \)-isomers is established. Then according to this comparison result, extremal hexagonal chains with maximum and minimum Kemeny's constant are characterized. It turns out that among all hexagonal chains with \( n \) hexagons, the linear chain \( L_n \) is the unique graph with the maximum Kemeny's constant, whereas the helicene chain \( H_n \) is the unique graph with the minimum Kemeny's constant.