Ultrasound with water as a green solvent is an effective strategy for reducing losses and increasing the utilization of by-products. The extraction of proteins and specialized plant metabolites from sugar beet leaves (Beta vulgaris subsp. vulgaris var. altissima) promotes sustainability in the agro-food chain. Guided by sustainability, samples treated with ultrasound showed lower energy consumption and lower CO2 emissions. Furthermore, the spectrophotometric determination revealed higher protein and phenol yields in ultrasonically treated samples compared to thermally treated ones. The highest yield of total proteins, 147.91 ± 4.58 mg (gd.m.)−1, was observed during ultrasound treatment (amplitude 100%, treatment time 9 min). Under the same extraction conditions, the same trend was observed in the yield of total phenols 17.89 ± 0.38 mg (gd.m.)−1. High-power ultrasound, compared to the thermal extraction method, has increased the yield of proteins and specialized plant metabolites with significantly lower energy consumption and CO2 emissions. The obtained results are in accordance with the foundations of sustainable development. From an economic and environmental point of view, ultrasound with the use of green solvents would be an excellent replacement for conventional extraction methods.