Life cycle assessment (LCA) is a powerful tool to quantify the environmental burdens of different analytical techniques. This work assesses the environmental impacts associated with the use of a simple electrochemical carbon paper sensor (CPS) for ketoprofen detection in fish by LCA in comparison with traditional liquid high-performance chromatography (HPLC) with fluorescent detection. The results indicate significant advantages of CPS compared to HPLC in 16 of the 18 analyzed categories of impact (ReCiPe2016(H) method), with average CPS values 26% lower than for HPLC. This is due, in the categories of impact with higher environmental relevance, to the higher electric energy consumption during the “Analysis” step and, secondarily, to the use of acetonitrile as a mobile phase. On an annual basis, ketoprofen detection by CPS saves 333 kg 1.4 dichlorobenzene equivalents (1.4 DCB eq) of non-carcinogenic and 6.9 kg 1.4 DCB eq of carcinogenic human toxicities, 43.6 kg oil eq of fossil resources, and 91.4 kg CO2 eq of greenhouse gas emissions compared to HPLC. The high capital investment, maintenance costs, and reagents quantities required for HPLC mitigate the economic competitiveness of this traditional technique compared to the rapid and less complex portable CPS device under the studied conditions.