This single-center study aimed to retrospectively evaluate the survival outcomes of patients with FIGO stage I clear cell and serous uterine carcinoma according to the type of adjuvant treatment received. The data were collected between 2003 and 2020 and only patients with stage I clear cell or serous uterine carcinoma treated with primary surgery were included. These were classified into three groups: No treatment or brachytherapy only (G1), radiotherapy +/− brachytherapy (G2), chemotherapy +/− radiotherapy +/− brachytherapy (G3). In total, we included 52 patients: 18 patients in G1, 16 in G2, and 18 in G3. Patients in the G3 group presented with poorer prognostic factors: 83.3% had serous histology, 27.8% LVSI, and 27.8% were FIGO stage IB. Patients treated with adjuvant radiotherapy showed an improved 5-year overall survival (OS) (p = 0.02) and a trend towards an enhanced 5-year progression-free survival (PFS) (p = 0.056). In contrast, OS (p = 0.97) and PFS (p = 0.84) in the chemotherapy group with poorer prognostic factors, were similar with increased toxicity (83.3%). Radiotherapy is associated with improved 5-year OS and tends to improve 5-year PFS in women with stage I clear cell and serous uterine carcinoma. Additional chemotherapy should be cautiously considered in serous carcinoma cases presenting poor histological prognostic factors.