Background: An association between the time from diagnosis to treatment initiation (TTI) and poorer overall survival (OS) has been found in multiple studies of early-stage cancers, including non-small cell lung cancer (NSCLC), head and neck squamous cell cancer (HNSCC). However, in several studies of metastatic NSCLC, a longer TTI was not associated with poorer OS. These results indicate that the association between TTI and OS may not be understood well. The objective of this study was to examine TTI and its determinants in metastatic cancer patients. Methods: This was a retrospective cohort study of Flatiron electronic health records from January 1, 2014-June 30, 2019. US adults newly diagnosed with metastatic NSCLC, HNSCC, and melanoma receiving first-line treatment within 365 days of diagnosis were included. Mean (SD) and median (Q1, Q3) TTI are reported by the unrestricted (all patients) pooled population, restricted (equal sampling from each cancer type) pooled population and individual cancer type. We also report determinants of TTI using generalized linear mixed effects model. Result: A total of 20,701 patients were included, of whom 17,392 (80.1%) had NSCLC, 1,869 (8.6%) had HNSCC, and 1,440 (6.6%) had melanoma. The mean (SD) age for the unrestricted pooled, restricted pooled, NSCLC, melanoma and HNSCC patients was 67.4 (10.2), 66.1 (11.1), 67.8 (9.9), 65.8 (13.2) and 64.4 (10.1) years respectively. The mean (SD) and median (Q1, Q3) TTI for the unrestricted pooled population were 48.7 (45.4) and 36.0 (24.0, 56.0) days, respectively. For the restricted pooled population: 62.5 (61.1) and 42.0 (27.0, 70.0) days. For NSCLC patients: 44.8 (39.1) and 35.0 (23.0, 52.0) days. For melanoma patients: 77.5 (77.4) and 47.0 (28.0, 93.5) days. For HNSCC patients: 62.3 (54.6) and 45.0 (29.0, 74.0) days. Significant determinants of TTI for the unrestricted pooled population were age, gender, charlson comorbidity index (CCI) and smoking status. For the restricted pooled population, no significant determinants were found. For NSCLC patients, age, gender, histology, CCI and smoking status were significant. Only gender was significant for HNSCC and none in the melanoma population. Conclusions: There is high TTI variability and also overlap between patients with metastatic NSCLC, HNSCC, and melanoma. Significant determinants of TTI depend on the cancer type.