Introduction: Appropriate patient selection based on functional status is crucial when considering older adults for palliative chemotherapy. This pre-planned analysis of the randomized NORDIC9-study explored the prognostic value of four functional status measures regarding progression-free survival (PFS) and overall survival (OS) in vulnerable older patients with metastatic colorectal cancer (mCRC) receiving first-line palliative chemotherapy. Materials and methods: Patients ≥70 years of age with mCRC not candidates for standard full-dose combination chemotherapy were randomized to receive full-dose S1 or reduced-dose S1 + oxaliplatin. At baseline, functional status was assessed using ECOG performance status (ECOG PS), frailty phenotype, Geriatric 8 (G8), and Vulnerable Elderly Survey-13 (VES-13). Multivariable regression models were applied and C-statistics were estimated. Results: In total, 160 patients with a median age of 78 years (IQR: 76–81) were included. While in univariate analyses, ECOG PS, frailty phenotype, and VES-13 were statistically significantly associated with differences in OS between subgroups, G8 was not (HR = 1.55, 95%CI: 0.99–2.41, p = 0.050). In multivariable analyses adjusted for age, sex, body mass index, and treatment allocation, we found significant differences between subgroups for all applied tools and with C-statistics in the moderate range for ECOG PS and VES-13. Concerning PFS, statistically significant differences were observed between subgroups of ECOG PS, G8, and VES-13 both in uni- and multivariable analyses, but not for frailty phenotype. Discussion: In this Nordic cohort of vulnerable older patients with mCRC, baseline ECOG PS, frailty phenotype, G8, and VES-13 showed prognostic value regarding overall survival, and moderate predictive value of models based on ECOG PS and VES-13 was demonstrated.