Systems and methods for performing online extracorporeal photopheresis of mononuclear cells are disclosed. During a mononuclear cell collection cycle, blood is removed from a source and separated into a plasma constituent, a mononuclear cell-containing layer, and red blood cells, followed by the collection of a pre-product including at least a portion of the mononuclear cell-containing layer and at least a portion of the separated red blood cells. The mononuclear cell collection cycle may be repeated, followed by the production of a single mononuclear cell product using the collected pre-product(s). The mononuclear cell product is irradiated using a fixed dose of light, such that the mononuclear cell product is produced so as to have a predetermined volume and a predetermined hematocrit, regardless of the number of pre-products used to produce the mononuclear cell product. Following irradiation, at least a portion of the irradiated mononuclear cell product is returned to the source.