Leucadendron sessile is a 1-2 m tall, rounded, dense, bushy shrub arising from a single stem at ground level, and usually branching low down. Branches are stout and short. Like all leucadendrons this species is dioecious, i.e. male and female flowers are borne on separate plants. The leaves are narrowly elliptical, up to 64 mm long in the male and up to 80 mm long in the female, hairless but not shiny, with red margins and tipped with a blunt, red, recurved point. The leaves increase in size and become more crowded towards the end of the stems.

Another characteristic of leucadendrons is the way the leaves that surround the inflorescence, known as the involucral leaves, turn a different colour while the plant is in flower. In Leucadendron sessile the involucral leaves are yellow reddening with age, and the male plants are usually a brighter yellow than the female. Flowering time is mid-winter to early spring (July to August).

In the genus Leucadendron the inflorescence consists of spirally arranged floral bracts and directly above each bract is a single flower. In the male inflorescence the floral bracts are small and inconspicuous, whereas in the female inflorescence the floral bracts are large and partially hide the flower. Individual leucadendron flowers are small and need a hand lens or dissecting microscope to be seen properly. The flowers consist of four perianth segments usually fused into a tube but with the tips free. At the base of the inflorescence there are basal bracts.

In Leucadendron sessile the male inflorescence is 23 mm long and 35 mm in diameter, shaped like a flattened globe and with a faint, sweet, lemony scent. Up to 50 brown to purplish brown, brittle basal bracts surround the base of the inflorescence. On some plants they are an attractive reddish brown in the buds. The flowers are yellow when fresh, ageing brown, and open in a spiral from the outside inwards. The fully open flower head is very attractive: a button made up of many dark yellow flowers surrounded by light yellow involucral bracts, sometimes suffused with red.

The female inflorescence is 21-28 mm long and 14-18 mm in diameter, shaped like an elongated globe and with a pleasant, sweet scent. About 30 basal bracts enclose the base of the inflorescence, closely overlapping, like roof tiles. They are usually brown to purplish brown and in some of the Kirstenbosch plants they are yellowish green in bud. In this species, the basal bracts hide the floral bracts, the flowers are crowded towards the tip of the inflorescence and only their tips stick out. Although not as bright as the male, the flower head is attractive: a yellow-tipped reddish brown inflorescence surrounded by light yellow or greenish yellow involucral bracts, usually suffused with red.

After flowering and pollination, the female inflorescence takes about four months to produce the fruits. The floral bracts continue growing, and become hard and woody, forming a cone, ± 40 mm long and ± 30 mm in diameter. The bracts are broadly elliptic, hairless above and covered in short, fine hairs below. The fruit is a hard nut, 85 mm long, 10 mm broad and 60 mm thick, hairless except for a ring of hairs around the basal scar, and drop to the ground in midsummer (December).

Both male and female flowers produce nectar from nectaries found inside the perianth tube at the base of the ovary and between each perianth segment, known as hypogynous scales, and thought to be modified petals. Both the male and female flowers contain the same number of floral parts, but in the male flower, the gynoecium (female part) is sterile, and in the female flower, the stamens are sterile.