The roles played by singlet oxygen (1O2) in photodynamic therapy are not fully understood yet. In particular, the mobility of 1O2 within cells has been a subject of debate for the last two decades. In this work, we report on the kinetics of 1O2 formation, diffusion, and decay in human skin fibroblasts. 1O2 has been photosensitized by two water-soluble porphyrins targeting different subcellular organelles, namely the nucleus and lysosomes, respectively. By recording the time-resolved near-IR phosphorescence of 1O2 and that of its precursor the photosensitizer's triplet state, we find that the kinetics of singlet oxygen formation and decay are strongly dependent on the site of generation. 1O2 photosensitized in the nucleus is able to escape out of the cells while 1O2 photosensitized in the lysosomes is not. Despite showing a lifetime in the microsecond time domain, 1O2 decay is largely governed by interactions with the biomolecules within the organelle where it is produced. This observation may reconcile earlier views that singlet oxygen-induced photodamage is highly localized, while its lifetime is long enough to diffuse over long distances within the cells.