Organic semiconductor lasers (OSLs) have emerged as particularly challenging. One of the major issues preventing the successful realization of lasing from organic emitters under electrically pumped conditions is the inevitable population of triplet excitons. Herein, a novel concept is presented to construct triplet–singlet guest–host gain systems with incorporating iridium complexes as the triplet sensitizers and a fluorescent conjugated polymer as the gain media to achieve light amplification. The direct triplet–singlet energy transfer process is confirmed by photoluminescence excitation spectra, photoinduced absorption spectroscopy, and fluorescence transients of the blend samples. Successful light amplification with a threefold lower amplified spontaneous emission threshold and much better lasing performance is demonstrated for the resulting triplet–singlet guest–host system as compared with the corresponding gain system without triplet sensitizers. Moreover, under electrically driven conditions, the fluorescent organic light-emitting diodes (OLEDs) based on the triplet–singlet guest–host systems with “triplet sensitizers” exhibit enhanced electrical performance relative to those without. The work suggests an effective general methodology to utilize both the singlet and triplet excitons to contribute to the light amplification with excellent electrical performance in OLEDs, opening prospects toward attempting electrically pumped OSLs.