IMPORTANCE Mixed urinary incontinence, including both stress and urgency incontinence, has adverse effects on a woman's quality of life. Studies evaluating treatments to simultaneously improve both components are lacking. OBJECTIVE To determine whether combining behavioral and pelvic floor muscle therapy with midurethral sling is more effective than sling alone for improving mixed urinary incontinence symptoms. DESIGN, SETTING, AND PARTICIPANTS Randomized clinical trial involving women 21 years or older with moderate or severe stress and urgency urinary incontinence symptoms for at least 3 months, and at least 1 stress and 1 urgency incontinence episode on a 3-day bladder diary. The trial was conducted across 9 sites in the United States, enrollment between October 2013 and April 2016; final follow-up October 2017. INTERVENTIONS Behavioral and pelvic floor muscle therapy (included 1 preoperative and 5 postoperative sessions through 6 months) combined with midurethral sling (n = 209) vs sling alone (n = 207). MAIN OUTCOMES AND MEASURES The primary outcome was change between baseline and 12 months in mixed incontinence symptoms measured by the Urogenital Distress Inventory (UDI) long form; range, 0 to 300 points; minimal clinically important difference, 35 points, with higher scores indicating worse symptoms. RESULTS Among 480 women randomized (mean [SD] age, 54.0 years [10.7]), 464 were eligible and 416 (86.7%) had postbaseline outcome data and were included in primary analyses. The UDI score in the combined group significantly decreased from 178.0 points at baseline to 30.7 points at 12 months, adjusted mean change −128.1 points (95% CI, −146.5 to −109.8). The UDI score in the sling-only group significantly decreased from 176.8 to 34.5 points, adjusted mean change −114.7 points (95% CI, −133.3 to −96.2). The model-estimated between-group difference (−13.4 points; 95% CI, −25.9 to −1.0; P = .04) did not meet the minimal clinically important difference threshold. Related and unrelated serious adverse events occurred in 10.2% of the participants (8.7% combined and 11.8% sling only). CONCLUSIONS AND RELEVANCE Among women with mixed urinary incontinence, behavioral and pelvic floor muscle therapy combined with midurethral sling surgery compared with surgery alone resulted in a small statistically significant difference in urinary incontinence symptoms at 12 months that did not meet the prespecified threshold for clinical importance.