Abstract The endolysosomal cysteine endoprotease cathepsin L is secreted from cells in a variety of pathological conditions such as cancer and arthritis. We compared the secretome composition and extracellular proteolytic cleavage events in cell supernatants of cathepsin L-deficient and wild-type mouse embryonic fibroblasts (MEFs). Quantitative proteomic comparison of cell conditioned media indicated that cathepsin L deficiency affects, albeit in a limited manner, the abundances of extracellular matrix (ECM) components, signaling proteins, and further proteases as well as endogenous protease inhibitors. Immunodetection corroborated that cathepsin L deficiency results in decreased abundance of the ECM protein periostin and elevated abundance of matrix metalloprotease (MMP)-2. While mRNA levels of MMP-2 were not affected by cathepsin L ablation, periostin mRNA levels were reduced, potentially indicating a downstream effect. To characterize cathepsin L contribution to extracellular proteolysis, we performed terminal amine isotopic labeling of substrates (TAILS), an N-terminomic technique for the identification and quantification of native and proteolytically generated protein N-termini. TAILS identified >1500 protein N-termini. Cathepsin L deficiency predominantly reduced the magnitude of collagenous cleavage sites C-terminal to a proline residue. This contradicts cathepsin L active site specificity and indicates altered activity of further proteases as a result of cathepsin L ablation.