In 2006, Seigler et. al. transferred this taxon from Acacia to the newly created genus Mariosousa. This was previously described in Prosopis, as Prosopis heterophylla in 1846, and as Acacia willardiana in 1890, so it is evident that they must have transferred it originally as M. heterophylla.
However, appealing to nomenclatural stability and the widespread use of the epithet "willardiana", they did so as M. willardiana, for which they proposed retaining A. willardiana over P. heterophylla in 2011, which was rejected.
Finally, in 2018 they formally published the current, and taxonomically more correct name.