Background: Combining lidocaine with bupivacaine in brachial plexus blocks seeks to blend rapid onset with extended duration; yet, clinical advantages are uncertain. This systematic review assesses their efficacy against bupivacaine alone in ultrasound-guided brachial plexus blocks. Methods: A systematic search of PubMed, EMBASE, and Cochrane databases was conducted in May 2025. Randomized controlled trials (RCTs) comparing lidocaine-bupivacaine mixtures with bupivacaine alone in ultrasound-guided brachial plexus blocks were included. The primary outcome was sensory block onset time. Secondary outcomes included motor block onset time, sensory and motor block durations, and conversion to general anesthesia. Data were analyzed using a random-effects model, with heterogeneity assessed via I² statistics. Results: Of 1,490 identified articles, 7 RCTs (358 patients) met the inclusion criteria. No significant difference was found in sensory block onset time (mean difference [MD] –1.81 min, 95% confidence interval [CI] –3.92 to 0.29; P = 0.09; I² = 98%) or motor block onset time (MD 0.02 min, 95% CI –2.34 to 2.39; P = 0.99; I² = 95%) between groups. The mixture reduced sensory (MD –172.88 min, 95% CI –215.18 to –130.59; P<0.00001; I² = 90%) and motor block durations (MD –212.13 min, 95% CI –374.99 to –49.28; P = 0.01; I² = 93%). Conclusions No clinical benefit was observed from combining lidocaine with bupivacaine, as there was no improvement in block onset times and a reduction in block durations. Given the very low certainty of evidence, these findings should be interpreted with caution, and further high-quality RCTs are needed.