We mechanise a proof of Green’s theorem in Isabelle/HOL. We use a novel proof that avoids the ubiquitous line integral cancellation argument. This eliminates the need to formalise orientations and region boundaries explicitly with respect to the outwards-pointing normal vector. Instead we appeal to a homological argument about equivalences between paths. Contributions include mechanised theories of line integrals and partial derivatives, as well as the first mechanisation of Green’s theorem.