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Hawthorn leaves are a rich source of phenolic compounds that possess beneficial activities for human health. Ultrasonic-assisted extraction (UAE) is an extraction technique frequently used for the isolation of phenolic compounds in plants. Thus, in this study, a Box–Behnken design was used to optimize UAE conditions such as the percentage of acetone, the extraction time and solvent-to-solid ratio (<i>v</i>/<i>w</i>) in order to obtain the maximum content of total compounds by Folin–Ciocalteu and the maximum in vitro antioxidant activity by DPPH, ABTS and FRAP assays in <i>Crataegus monogyna</i> leaves. The optimum conditions to obtain the highest total phenolic content and antioxidant activities were 50% acetone, 55 min and 1/1000 (<i>w</i>/<i>v</i>). A total of 30 phenolic compounds were identified and quantified in <i>C. monogyna</i> leaf extract obtained at these optimum UAE conditions. HPLC-MS allows the identification and quantification of 19 phenolic compounds and NP-HPLC-FLD analyses showed the presence of 11 proanthocyanidins. According to the results, the most concentrated phenolic compounds in <i>C. monogyna</i> leaf extract obtained at optimum UAE conditions were phenolic acid derivatives such as protocatechuic acid-glucoside, dihydroxy benzoic acid pentoside and chlorogenic acid, flavones such as 2″-<i>O</i>-rhamnosyl-<i>C</i>-hexosyl-apigenin, flavonols such as hyperoside and isoquercetin and proanthocyanidins such as monomer and dimer. As a result, the optimized UAE conditions could be used to obtain an extract of <i>C. monogyna</i> leaves enriched with phenolic compounds.