Myocardial infarction (MI) and atrial fibrillation (AF) are commonly seen in the same patient. In this study, we evaluated the temporal relations and prognosis of MI and AF. This is a substudy of the nationwide registry–based Finnish Anticoagulation in Atrial Fibrillation (FinACAF) study, comprising all Finnish patients with new-onset AF from 2010 to 2017. Patients with MI and AF were divided into groups depending on the temporal relation between the disease onsets: (1) MI before AF (MI<AF), (2) MI ± 30 days before or after AF (MI=AF), (3) MI after AF (MI>AF), and (4) no MI. The 1-year mortality in the groups were studied with the Cox proportional hazards model. Of the 153,207 patients with new-onset AF (mean age 72.7 years, 50.0% women), 16,265 (10.6%) were diagnosed with MI. Altogether, 8,889 (54.7%) of the patients with MI were in the MI<AF group, 4,278 (26.3%) were in the MI=AF group, and 3,098 (19.1%) were in the MI>AF group. Of all MIs, 42.2% were diagnosed within 1 year from new-onset AF. The MI>AF group had the worst survival, with an adjusted hazard ratio for death of 3.08 (confidence interval [CI] 2.89 to 3.27) compared with patients without MI. For the MI<AF and MI=AF groups, the hazard ratios were 1.34 (CI 1.27 to 1.41) and 1.69 (CI 1.59 to 1.81). In conclusion, the diagnoses of MI and AF accumulated close to one another, and the survival of patients with concomitant AF and MI varied, with the worst outcome found in patients with MI diagnosed after the new-onset AF.