The evolution of mesoscale eddies is crucial for understanding the ocean energy cascade. In this study, using global reanalysis sea surface velocity data and a mesoscale eddy trajectory product tracked by satellite altimeters, we aimed to reveal the coherence of eddy kinetic energy (EKE) variation to low-frequency ageostrophic energy during the eddy life span. The variation in EKE throughout the eddy life span was highly coherent to that of the seven-day low-passed ageostrophic kinetic energy, with a correlation coefficient of −0.94. The low-frequency ageostrophic motions supplied 38% of the EKE variation in the growing stage of mesoscale eddies and absorbed 42% in the decaying stage. The evolution rate of the iEKE time derivatives during the eddy life span was consistent with the barotropic conversion rate of the low-frequency ageostrophic motions, further confirming the dominant role of low-frequency ageostrophic motions in eddy growth and decay.