In this article we use cluster structures and mirror symmetry to explicitly describe a natural class of Newton-Okounkov bodies for Grassmannians. We consider the Grassmannian X = Gr n−k (C n ), as well as the mirror dual Landau-Ginzburg model (X ○ , W ∶X ○ → C), whereX ○ is the complement of a particular anti-canonical divisor in a Langlands dual GrassmannianX = Gr k ((C n ) * ), and the superpotential W has a simple expression in terms of Plücker coordinates [MR13]. Grassmannians simultaneously have the structure of an A-cluster variety and an X -cluster variety [Sco06, Pos]; roughly speaking, a cluster variety is obtained by gluing together a collection of tori along birational maps [FZ02, FG06]. Given a plabic graph or, more generally, a cluster seed G, we consider two associated coordinate systems: a network orare the open positroid varieties in X andX, respectively. To each X -cluster chart Φ G and ample 'boundary divisor' D in X ∖ X ○ , we associate a Newton-Okounkov body ∆ G (D) in R k(n−k) , which is defined as the convex hull of rational points; these points are obtained from the multi-degrees of leading terms of the Laurent polynomials Φ * G (f ) for f on X with poles bounded by some multiple of D. On the other hand using the A-cluster chart Φ ∨ G on the mirror side, we obtain a set of rational polytopes -described in terms of inequalities -by writing the superpotential W as a Laurent polynomial in the A-cluster coordinates, and then "tropicalising". Our first main result is that the Newton-Okounkov bodies ∆ G (D) and the polytopes obtained by tropicalisation on the mirror side coincide. As an application, we construct degenerations of the Grassmannian to normal toric varieties corresponding to (dilates of) these Newton-Okounkov bodies. Our second main result is an explicit combinatorial formula in terms of Young diagrams, for the lattice points of the Newton-Okounkov bodies, in the case that the cluster seed G corresponds to a plabic graph. This formula has an interpretation in terms of the quantum Schubert calculus of Grassmannians [FW04].