Functionalizing the organic building blocks of electrically conductive MOFs (EC-MOFs) can be a powerful method for adjusting the electronic structure and introducing a specific chemistry. However, designing EC-MOF linkers with reactive functional groups for postsynthetic modification is challenging due to the requirements of d-p conjugation. This work addresses such design limitations by synthesizing an EC-MOF, Cu-thiatruxene (Cu-thiaTRX). This conductive framework incorporated a truxene-based linker with heterocyclic sulfur, allowing for efficient conjugation and an electrical conductivity of 2.2 × 10-2 S cm-1. Harnessing sulfur chemistry in Cu-thiaTRX involves a two-step postsynthetic modification: oxidation and SNAr. The sulfinic groups introduced in the framework enabled tunable proton conductivity, leading to a 200-fold improvement. These results highlight the importance of a rational linker design for functionalization.