The mulga complex (Acacia aneura F. Muell ex Benth and closely related species) consists of woody trees and shrubs, and is distributed across 20% of the Australian continent. A. aneura is renowned for a wide variety of phyllode shapes and growth forms, which may co-occur at any one site. We examined the intra- and inter-specific variation in growth form and phyllode shape in four species of the mulga complex, including A. aneura, across topographic gradients in semiarid north-west Australia. We measured 792 trees across 28 sites stratified into six discrete landscape positions; upper slope, lower slope, low open woodland, banded woodland, low woodland, and drainage line. Dominance of phyllode shapes was strongly related to landscape position. A. aneura with terete phyllodes were dominant on the hill slopes, whereas broad phyllodes were most common on A. aneura in all valley woodlands. Trends in growth form were less distinct, although single-stemmed forms were more common on hills, whereas the valleys had more multi-stemmed forms. The quantification of growth form and phyllode shape variability within the mulga complex provides a basis for the quantitative determination of functional links between morphology and environmental conditions at both the site and landscape level.