This herbaceous perennial (i.e. with stems which die down in the autumn) grows to 180 cm (6 ft) tall and puts out only a few lateral branches. The whole plant is softly stellate-hairy, especially the leaves, which are broadly triangular to oval, often with 3-5 shallow lobes, irregularly toothed, with cordate to cuneate bases. Leaf size varies considerably, up to 100 mm (4 in) long, and 75 mm (3 in) wide. The leaves are arranged alternately along the stem, with no stipules, on petioles up to 45 mm (1.75 in).

The inflorescences occur in the leaf axils and at the top of the stem and consist of panicles of 1-many flowers. The flowers are actinomorphic with 5 lilac/pink petals up to 2 cm long and 5 green sepals which are much shorter than the petals, and fused at the base. Below the petals is a cup-shaped epicalyx with 6-9 narrow, triangular lobes, half the length of the sepals. The purple stamens are united into a tube, the anthers kidney-shaped and one-celled. There is one style which protrudes above the stamen tube.

The flowers are in bloom during August and September, and are followed, as in other species of this order, by the flat, round fruit which are popularly called "cheeses". The whole fruit is a schizocarp, about 1 cm in diameter, which splits into about 20 kidney-shaped mericarps (seeds) about 2 mm long.

The common mallow is frequently called "marsh mallow" in colloquial terms, but the true marsh mallow is distinguished from all the other mallows growing in Great Britain by the numerous divisions of the outer calyx (six to nine cleft), by the hoary down which thickly clothes the stems and foliage, and by the numerous panicles of blush-coloured flowers, paler than the common mallow. The roots are perennial, thick, long and tapering, very tough and pliant, whitish yellow outside, white and fibrous within.