Distributed architectures are used to improve performance and reliability of various systems. An important capability of a distributed architecture is the ability to reach consensus among all its nodes. To achieve this, several consensus algorithms have been proposed for various scenarii, and many of these algorithms come with proofs of correctness that are not mechanically checked. Unfortunately, those proofs are known to be intricate and prone to errors. In this paper, we formalize and mechanically check a consensus algorithm widely used in the distributed controls community: the Weighted-Mean Subsequence Reduced (W-MSR) algorithm proposed by Le Blanc et al. This algorithm provides a way to achieve asymptotic consensus in a distributed controls scenario in the presence of adversarial agents (attackers) that may not update their states based on the nominal consensus protocol, and may share inaccurate information with their neighbors. Using the Coq proof assistant, we formalize the necessary and sufficient conditions required to achieve resilient asymptotic consensus under the assumed attacker model. We leverage the existing Coq formalizations of graph theory, finite sets and sequences of the mathcomp library for our development. To our knowledge, this is the first mechanical proof of an asymptotic consensus algorithm. During the formalization, we clarify several imprecisions in the paper proof, including an imprecision on quantifiers in the main theorem.