Recently, photodynamic therapy (PDT) has found wide application as a noninvasive treatment modality for several cancers. However, the suboptimal delivery of photosensitizers (PSs) to the tumor site is a drawback, which inhibits the effectiveness of PDT. Hydrophobicity, strong oxygen and light dependence, and limited tissue penetrability of photosensitizers represent the major barriers to the clinical application of PDT. In order to improve biopharmaceutical properties of a clinically approved photosensitizer chlorin e6 (Ce6), we developed a nanoformulation encapsulating Ce6 in methoxy-poly(ethylene glycol)-poly(d,l-lactide) (mPEG-PLA) copolymeric micelles. The physicochemical properties, including particle size, zeta potential, encapsulation efficiency, drug loading, generation of reactive oxygen species following near-infrared light illumination (633 nm), and in vitro drug release, were determined. The therapeutic efficacy of Ce6-mPEG-PLA micelles following illumination were evaluated in vitro in both two- and three-dimensional cell culture systems by using human uterine cervix carcinoma (HeLa) and human alveolar adenocarcinoma (A549) cells in monolayers and in A549 spheroids, respectively. The mPEG-PLA micelles were stable with a particle size of 189.6 ± 14.32 nm and loaded Ce6 efficiently (encapsulation efficiency ∼75%). The Ce6-loaded micelles generated singlet oxygen at a higher concentration compared to free Ce6 in aqueous media. Ce6-mPEG-PLA micelle mediated PDT showed improved cellular internalization in both of the cell lines, resulting in enhanced cytotoxicity compared to free Ce6. In contrast, the Ce6-loaded micelles did not show any cytotoxicity in the absence of irradiation. The Ce6-loaded micelles exhibited deep penetration in the spheroids leading to phototoxicity and cellular apoptosis in the A549 spheroidal model. Results from this study indicated that the newly developed nanoformulation of Ce6 could be utilized in PDT as an effective treatment modality for solid tumors.