Cell surface mesothelin (MSLN) can be solubilized and released into the systemic circulation. The resulting soluble MSLN (sMSLN) may interfere with therapies targeting surface MSLN. We investigated the effects of sMSLN on anetumab, an antibody-based therapy against MSLN, anetumab ravtansine, an antibody drug conjugate, and mechanisms to decrease sMSLN. Whole blood samples were collected before and after one plasma volume of therapeutic plasma exchange (TPE). sMSLN levels were measured with ELISA assays in matched pre- and post-TPE plasma samples, and anetumab-immunoprecipitated samples. We also used protease inhibitors (PIs) as a mechanism to stabilize surface MSLN, then evaluated the cytotoxic effects of anetumab ravtansine. Our findings indicate that sMSLN sequesters and may impair the efficacy of this anti-MSLN antibody based on results showing that anetumab decreases the concentration of MSLN in plasma (p < 0.05) and reduced cytotoxicity of anetumab ravtansine in the presence of recombinant MSLN in cell lines, a surrogate for sMSLN. TPE consistently reduced sMSLN (p < 0.05) with an average decrease of 43.6% (15.4 ng/mL). Surface MSLN stabilization was inconsistently observed with PIs. Overall, sMSLN could represent a predictive biomarker for MSLN directed therapies. TPE may be more reliable than PIs to reduce sMSLN and ultimately restore sensitivity to these therapies in patients with high sMSLN.