The sexes are similar except for the female being a shade larger, with larger orange markings and a warmer reddish brown over the upper wings. Upper wings are both sooty brown with 4 - 4 orange marginal lunules (spots). The hindwings have five, each centre dotted with black. The forewings have a narrow black margin followed by a white margin at the outermost edge, which is slightly segmented by black veins. There is a black discal spot on the forewings, which is more obvious on the female. The underwings are silvery at a casual glance. The ground colour is greyish or greyish-brown, with seven white ringed black dots on the forewings, and the outer margins are lined with bold orange dots. On the hindwings there is another scattering of black dots surrounded by white. The particular arrangement of these is vital to distinguish this species from brown examples of the female Common Blue.
The first generation butterflies emerge in May and June, the second generation are seen in July to September, and in warm autumns into October, but the flight season is normally about 3 weeks.
The Brown Argus is very fond of Marjoram, where it will be found with a variety of other species, especially Common Blue; also Bird's-foot Trefoil, and Ragwort. In spring the nectar sources may be Milkwort, and hawkweeds.