Endometrial cancer (EC) patients make up the second largest group of female cancer survivors. Patient-reported outcomes (PROs) including quality of life (QOL) and sexual function and satisfaction (SF and S) are critical facets of survivorship. This prospective, longitudinal study assesses associations between baseline characteristics and PROs after treatment. Herein, we report the baseline clinical characteristics and PROs prior to treatment initiation. Outcomes post-treatment over time will be reported separately. Patients with planned surgery for EC were prospectively enrolled in 2019–2021 and administered the European Organization for Research and Treatment of Cancer (EORTC) QOL Questionnaire Core 30 (QLQ-C30), EORTC QLQ EC Module (EN24), Patient-Reported Outcomes Measurement Information System (PROMIS), and the Mayo Clinic lower extremity lymphedema (LEL) questionnaire. This study enrolled 198 patients with a mean (SD) age of 63.6 (9.8) years and body mass index of 35.5 (8.3) kg/m². No significant differences in the PROs for the QOL were seen when compared to the reference means (SD) except for the lower interest in sexual activity (31.9 (9.8) vs. 47.5 (SE0.70)) and lower fatigue (21.3 (19.8) vs. 31.7 (25.9)). Increased obesity was associated with an increased likelihood of LEL (p < 0.01) and multiple QOL scales, including poorer global health status (p < 0.01) and physical functioning (p < 0.01). Prior to treatment initiation for EC, the patients had a similar QOL compared to that of the general population. The patients with increasing obesity, a known risk factor for EC, had worse overall global health status and physical functioning. A better understanding of these QOL measures is imperative for earlier identification and intervention of patients at risk of chronic impairments from EC treatment.