The present work deals with the extraction of phenolic compounds from biomass through microwave-assisted processes. In this case, green walnut shell residues have being studied for the extraction of phenolic compounds with antioxidant power, as an attractive option for their valorization toward value-added compounds. Thus, our work is focused on the optimization of the microwave extraction conditions, in terms of solvent system, temperature, and time, in order to obtain phenolic compounds with higher antioxidant properties from the green walnut shell. Several tests were performed to investigate the effect of each parameter on the antioxidant activity and in the total phenolic content. Water/acetonitrile (1:1) was found as the best solvent system. In addition, 30 min and 100 °C were selected as optimum extraction parameters. Under the aforementioned conditions, 95% of antioxidant power and a total phenolic content 99.09 ± 0.09 mg GAE/g of extract was achieved. These results are in accordance with the observed data from ATR-IR spectra, suggesting that under such conditions the best balance between phenolic content and lower polysaccharide contamination was reached. This work could pave the way in order to minimize the impact of waste on the environment, and it can be useful for those companies that would like to introduce the circular economy and obtain more benefits from the generated residues.