Patients with intellectual and developmental disabilities (IDD) face unique challenges resulting in disparities in their health care. We sought to define the effect that IDD had on achievement of a "textbook outcome" (TO) following a cancer operation among a nationally representative cohort of patients. Data on patients who underwent surgery for a malignant indication, including lung, breast, liver, biliary tract, pancreas, and colorectal, between 2014 and 2020 were extracted from the 100% Medicare Standard Analytical Files database. The association of IDD with TO (defined as the absence of postoperative complications, extended length of stay, 90-day readmission, and 90-day mortality), expenditures, and discharge status was assessed using multivariable logistic regression. Among 500,472 Medicare beneficiaries, 4,326 (0.9%) with IDD had a cancer diagnosis (breast, n=481; lung, n=419; hepatobiliary, n=194; pancreas, n=145; colorectal, n=3,087). Although overall incidence of TO was 50.5%, patients with IDD were less likely to achieve a TO than those without (37.1% vs 50.6%, respectively; odds ratio [OR], 0.50; 95% CI, 0.46-0.53; P<.001). On multivariable regression, patients with IDD had higher odds of a postoperative complication (OR, 1.53; 95% CI, 1.43-1.64), extended length of stay (OR, 2.06; 95% CI, 1.93-2.21), 90-day readmission (OR, 1.15; 95% CI, 1.07-1.24), 90-day mortality (OR, 1.90; 95% CI, 1.70-2.13), and discharge to a skilled nursing facility (OR, 4.28; 95% CI, 3.97-4.62) (all P<.001). Patients with IDD had a much lower chance of a postoperative TO, as well as discharge to a nonhome setting. The data highlight the need to improve the care of patients with IDD to assure equitable oncologic surgical care.