Common mallow is a herbaceous perennial with an erect or decumbent branched stem up to 1 m (sometimes 1.5 m) long. The minutely-ridged stems are covered with fine soft hairs, sometimes with a slightly bulbous base. The leaves are alternate, with a petiole up to 20 cm long, simple but palmate, up to 7 cm long by 10 cm wide, with stellate hairs (i.e. several strands radiating from a common center) and prominent veins on the underside.

The flowers are reddish-purple with dark stripes and occur in axillary clusters of 2 to 4 along the main stem with the flowers at the base opening first. There are 5 petals, each being up to 20 mm long, and 5 sepals, 3-6 mm long, which are fused in the lower half, and have broadly triangular lobes. There is also an epicalyx (or false calyx) with oblong segments, two-thirds as long as calyx lobes (2–3 mm long x 1.5 mm wide).