Mechanical verification of a constructive proof for FLP. The impossibility of distributed consensus with one faulty process is a result with important consequences for real world distributed systems e.g., commits in replicated databases. Since proofs are not immune to faults and even plausible proofs with a profound formalism can conclude wrong results, we validate the fundamental result named FLP after Fischer, Lynch and Paterson by using the interactive theorem prover Isabelle/HOL. We present a formalization of distributed systems and the aforementioned consensus problem. Our proof is based on Hagen Völzer’s paper “A constructive proof for FLP”. In addition to the enhanced confidence in the validity of Völzer’s proof, we contribute the missing gaps to show the correctness in Isabelle/HOL. We clarify the proof details and even prove fairness of the infinite execution that contradicts consensus. Our Isabelle formalization may serve as a starting point for similar proofs of properties of distributed systems.