Importance: Data regarding outcomes after major head and neck ablation and reconstruction in the growing geriatric population (specifically ≥80 years of age) are limited. Such information would be extremely valuable in preoperative discussions with elderly patients about their surgical risks and expected functional outcomes. Objectives: To identify patient and surgical factors associated with 30-day postoperative complications, 90-day mortality, and 90-day functional decline; to explore whether an association exists between the type of reconstructive procedure and outcome; and to create a preoperative risk stratification system for these outcomes. Design, Setting, and Participants: This retrospective, multi-institutional cohort study included patients 80 years or older undergoing pedicle or free-flap reconstruction after an ablative head and neck surgery from January 1, 2015, to December 31, 2017, at 17 academic centers. Data were analyzed from February 1 through April 20, 2019. Main Outcomes and Measures: Thirty-day serious complication rate, 90-day mortality, and 90-day decline in functional status. Preoperative comorbidity and frailty were assessed using the American Society of Anesthesiologists classification, Adult Comorbidity Evaluation-27 score, and Modified Frailty Index. Multivariable clustered logistic regressions were performed. Conjunctive consolidation was used to create a risk stratification system. Results: Among 376 patients included in the analysis (253 [67.3%] men), 281 (74.7%) underwent free-flap reconstruction. The median age was 83 years (range, 80-98 years). A total of 193 patients (51.3%) had 30-day serious complications, 30 (8.0%) died within 90 days, and 36 of those not dependent at baseline declined to dependent status (11.0%). Type of flap (free vs pedicle, bone vs no bone) was not associated with these outcomes. Variables associated with worse outcomes were age of at least 85 years (odds ratio [OR] for 90-day mortality, 1.19 [95% CI 1.14-1.26]), moderate or severe comorbidities (OR for 30-day complications, 1.80 [95% CI, 1.34-2.41]; OR for 90-day mortality, 3.33 [95% CI, 1.29-8.60]), body mass index (BMI) of less than 25 (OR for 30-day complications, 0.95 [95% CI, 0.91-0.99]), high frailty (OR for 30-day complications, 1.72 [95% CI, 1.10-2.67]), duration of surgery (OR for 90-day functional decline, 2.94 [95% CI, 1.81-4.79]), flap failure (OR for 90-day mortality, 3.56 [95% CI, 1.47-8.62]), additional operations (OR for 30-day complications, 5.40 [95% CI, 3.09-9.43]; OR for 90-day functional decline, 2.94 [95% CI, 1.81-4.79]), and surgery of the maxilla, oral cavity, or oropharynx (OR for 90-day functional decline, 2.51 [95% CI, 1.30-4.85]). Age, BMI, comorbidity, and frailty were consolidated into a novel 3-tier risk classification system. Conclusions and Relevance: Important demographic, clinical, and surgical characteristics were found to be associated with postoperative complications, mortality, and functional decline in patients 80 years or older undergoing major head and neck surgery. Free flap and bony reconstruction were not independently associated with worse outcomes. A novel risk stratification system is presented.