Polymer-based microwell platforms have garnered much interest due to their usefulness in culturing and analyzing small quantities of biological cells and spheroids. Existing methods for fabricating polymer microwell arrays involve complex fabrication processes and/or are limited in their ability to create dense arrays of very small (<50 μm in diameter) microwells. Here, we present a simple and rapid technique for fabricating high-density arrays of microwells ranging from 20 to 160 μm in diameter on a variety of polymer substrates. In this approach, a polymer surface is ablated using a CO2 laser that is rastered over a stainless steel mesh, which serves as a shadow mask. A theoretical laser-polymer interaction model was developed for predicting the microwell volume based on the substrate properties and laser settings. Microwell volumes predicted by the model were within 5.4% of fabricated microwell volumes determined experimentally. Cellulose acetate microwell arrays fabricated using this technique were used to culture Lewis lung carcinoma cells expressing ovalbumin (LLC-OVA), which were maintained for up to 72 h with a negligible (<5%) loss in viability. As a second proof of principle demonstration, LLC-OVA cells grown in microwell arrays were co-cultured with OT-I T cells and measurements of interferon gamma (IFN-γ), a marker for T cell activation, were performed which revealed a positive correlation between LLC-OVA cell-T cell interaction time and T cell activation. These two in vitro demonstrations showcase the capability of this technique in generating polymer microwell arrays for high-throughput cellular studies, including cell growth dynamics studies and cell interaction studies. Furthermore, we envision that these platforms can be used with different cell types and for other biological applications, such as spheroid formation and single cell analysis, further expanding the utility of this technique.