Mulga is highly variable, in form, in height, and in shape of phyllodes and seed pods. It can form dense forest up to 15 metres high, or small, almost heath-like low shrubs spread well apart. Most commonly, it is a tall shrub. Because it is so variable, the taxonomy of the Mulga has been studied extensively, and although it is likely to be split into several species eventually, there is as yet no consensus on how or even if this should be done. Although generally small in size, Mulga is long-lived, a typical life span for a tree undisturbed by fire is in the order of 200 to 300 years.
Mulga has developed extensive adaptations to the Australian desert. Like many Acacia species, Mulga has thick-skinned phyllodes. These are optimised for low water loss, with a high oil content, sunken stomata, and a profusion of tiny hairs to reduce transpiration. During dry periods, a Mulga drops much of its foliage to the ground, which provides an extra layer of mulch and from where the nutrients can be recycled.
Like most Australian Acacia species, mulga is thornless. The needle-like phyllodes stand erect to avoid as much of the midday sun as possible and capture the cooler morning and evening light. Any rain that falls is channeled down the phyllodes and branches to be collected in the soil immediately next to the trunk, providing the tree with a more than threefold increase in effective rainfall. Mulga roots penetrate far into the soil to find deep moisture. The roots also harbour bacteria that fix atmospheric nitrogen and thus help deal with the very old, nutrient poor soils the species grows in.