INTRODUCTION: Older women with early invasive breast cancer (EIBC) are more likely to receive a mastectomy compared with younger women. This study assessed factors associated with receiving a mastectomy among older women with EIBC, with a particular focus on comorbidity and frailty. MATERIALS AND METHODS: Women diagnosed with EIBC (stages I-IIIa) aged ≥50 years from 2014 to 2019 in English and Welsh NHS organisations who received breast surgery were identified from cancer registration datasets linked to routine hospital data. Separate multivariable logistic regression models explored factors associated with mastectomy use, within each tumour stage (T1-T3). For each tumour stage, risk-adjusted rates of mastectomy were calculated for each NHS organisation and displayed using funnel plots. RESULTS: We included 106,952 women with EIBC: 23.4% received a mastectomy as their first breast cancer surgery. Receipt of mastectomy was more common among patients with a higher tumour stage (T1: 12.3%; T2: 37.6%; T3: 77.5%), and mastectomy use increased with age within each tumour stage category (50-59 vs 80 + years: 11.8% vs 26.3% for T1; 31.5% vs 56.9% for T2; 73.4% vs 90.3% for T3). Results from a multivariable regression model showed that more severe frailty was associated with mastectomy use for women with T1 (p = 0.002) or T2 (p = 0.003) tumours, but may not be for women with T3 tumours (p = 0.041). There was no association between comorbidity and mastectomy use after accounting for frailty (all p > 0.1). Adjusting for clinical and patient factors only slightly reduced the association between age and mastectomy use. Variation in mastectomy use between NHS organisations was greatest for women with T2 EIBC (unadjusted range: 17.7% to 68.4%). DISCUSSION: Older women with EIBC are more commonly treated with mastectomy. This could not be explained by tumour characteristics or physical fitness, raising questions about whether surgical decision-making inconsistently incorporates information on patient fitness and functional age.