(1) Background: Longitudinal changes in myocardial T1 relaxation time are unknown. We aimed to assess the longitudinal changes in the left ventricular (LV) myocardial T1 relaxation time and LV function. (2) Methods: Fifty asymptomatic men (mean age, 52.0 years) who underwent 1.5 T cardiac magnetic resonance imaging twice at an interval of 54 ± 21 months were included in this study. The LV myocardial T1 times and extracellular volume fractions (ECVFs) were calculated using the MOLLI technique (before and 15 min after gadolinium contrast injection). The 10-year Atherosclerotic Cardiovascular Disease (ASCVD) risk score was calculated. (3) Results: No significant differences in the following parameters were noted between the initial and follow-up assessments: LV ejection fraction (65.0 ± 6.7% vs. 63.6 ± 6.3%,p= 0.12), LV mass/end-diastolic volume ratio (0.82 ± 0.12 vs. 0.80 ± 0.14,p= 0.16), native T1 relaxation time (982 ± 36 vs. 977 ± 37 ms,p= 0.46), and ECVF (24.97 ± 2.38% vs. 25.02 ± 2.41%,p= 0.89). The following parameters decreased significantly from the initial assessment to follow-up: stroke volume (87.2 ± 13.7 mL vs. 82.6 ± 15.3 mL,p= 0.01), cardiac output (5.79 ± 1.17 vs. 5.50 ± 1.04 L/min,p= 0.01), and LV mass index (110.16 ± 22.38 vs. 104.32 ± 18.26 g/m2,p= 0.01). The 10-year ASCVD risk score also remained unchanged between the two timepoints (4.71 ± 0.19% vs. 5.16 ± 0.24%,p= 0.14). (4) Conclusion: Myocardial T1 values and ECVFs were stable over time in the same middle-aged men.