Seurat spent over two years painting <em>A Sunday Afternoon</em>, focusing meticulously on the landscape of the park. He reworked the original as well as completed numerous preliminary drawings and oil sketches. He would go and sit in the park and make numerous sketches of the various figures in order to perfect their form. He concentrated on the issues of colour, light, and form. 
Motivated by study in optical and colour theory, Seurat contrasted miniature dots of colors that, through optical unification, form a single hue in the viewer's eye. He believed that this form of painting, now known as pointillism, would make the colors more brilliant and powerful than standard brush strokes. To make the experience of the painting even more vivid, he surrounded it with a frame of painted dots, which in turn he enclosed with a pure white, wooden frame, which is how the painting is exhibited today at the Art Institute of Chicago.
In creating the picture, Seurat employed the then-new pigment zinc yellow (zinc chromate), most visibly for yellow highlights on the lawn in the painting, but also in mixtures with orange and blue pigments. In the century and more since the painting's completion, the zinc yellow has darkened to brown—a colour degeneration that was already showing in the painting in Seurat's lifetime.