The ferromagnetic transition in (Zn,Sn,Mn)As2 thin films is explained in terms of magnetic percolation in a Mn-As clustering network. We first studied the relationship between the spatial distribution of Mn-As clusters and the Curie temperature (TC). The local atomic structure was reconstructed from datasets of atomic positions in (Zn,Sn,Mn)As2 obtained by atom probe tomography (APT). To probe the local atomic structure and the magnetic properties of Mn-As clusters in ZnSnAs2 thin films, we investigated molecular beam epitaxially grown (Zn,Sn,Mn)As2 samples doped with 2.1 and 3.6 at. % Mn. Representative regions with a low and high Mn concentration were extracted from APT datasets. Mn-As clusters containing 2-36 Mn atoms were identified in regions of high Mn concentration. We also obtained a correlation between TC and Mn-As clustering that was consistent with not only the experimental results but also first-principles calculations using the mean-field approximation.