The forward conversion from singlets to triplets and backward conversion from triplets to singlets are both possible in charge-transfer (CT) fluorescence-based organic light-emitting devices (OLEDs) due to the small energy difference between singlet and triplet CT states. Thus, clarifying factors affecting the direction of the conversion is important to obtain highly efficient OLEDs based on thermally activated delayed fluorescence (TADF). Here, we investigated the inter-conversion between singlets and triplets in a CT fluorescence-based OLED via magneto-electroluminescence (MEL) measurements. The values of MELs turned from negative to positive with the increase in driving voltage and electric stressing time, indicating that the direction of the conversion was changed. Both the increase in driving voltage and stressing time can cause the quenching of triplets through triplet–triplet annihilation and triplet–trap interaction, respectively. Thus, the decreased population ratio of triplet to singlet induces the change of the direction of inter-conversion. Our results demonstrate that the singlet–triplet inter-conversion is a dynamic process, and the deactivation rates of singlet and triplet determine the inter-conversion direction.