Background: Older people are the largest group admitted to hospital with serious injuries. Many older people are living with frailty, a risk factor for poor recovery. We aimed to examine the effect of preinjury frailty on outcomes. Methods: In this multicentre observational study (FiTR 1), we extracted prospectively collected data from all 23 adult major trauma centres in England on older people (aged ≥65 years) admitted with serious injuries over a 2·5 year period from the Trauma Audit and Research Network (TARN) database. Geriatricians assessed the preinjury Clinical Frailty Scale (CFS), a 9-point scale of fitness and frailty, with a score of 1 indicating a patient is very fit and a score of 9 indicating they are terminally ill. The primary outcome was inpatient mortality, with patients censored at hospital discharge. We used a multi-level Cox regression model fitted with adjusted hazards ratios (aHRs) to assess the association between CFS and mortality, with CFS scores being grouped as follows: a score of 1–2 indicated patients were fit; a score of 3 indicated patients were managing well; and a score of 4–8 indicated patients were living with frailty (4 being very mild, 5 being mild, 6 being moderate, and 7–8 being severe). Findings: Between March 31, 2019, and Oct 31, 2021, 193 156 patients had records were held by TARN, of whom 16 504 had eligible records. Median age was 81·9 years (IQR 74·7–88·0), 9200 (55·7%) were women, and 7304 (44·3%) were men. Of 16 438 patients with a CFS score of 1–8, 11 114 (67·6%) were living with frailty (CFS of 4–8). 1660 (10·1%) patients died during their hospital stay, with a median time from admission to death of 9 days (IQR 4–18). Compared in patients with a CFS score of 1–2, risk of inpatient death was increased in those managing well (CFS score of 3; aHR 1·82 [95% CI 1·39–2·40]), living with very mild frailty (CFS score of 4: 1·99 [1·51–2·62]), living with mild frailty (CFS score of 5: 2·61 [1·99–3·43]), living with moderate frailty (CFS score of 6: 2·97 [2·26–3·90]), and living with severe frailty (CFS score of 7–8: 4·03 [3·04–5·34]). Interpretation: Our findings support inclusion of the CFS in trauma pathways to aid patient management. Additionally, people who exercise regularly (CFS of 1–2) have better outcomes than those with lower activity levels (CFS of ≥3), supporting exercise as an intervention to improve trauma outcomes. Funding: None.