It is necessary to look at the spiders' copulatory organs to distinguish them from others in the genus, particularly the shorter length of the female's insemination ducts and the size of the palpal bulb.

Pachyballus mombasensis is a species of jumping spider that was first described by the arachnologists Wanda Wesołowska, Galina Azarkina and Konrad Wiśniewski in 2020.

[4] In their 2003 phylogenetic analysis, Wayne Maddison and Marshall Hedin noted that the genus is closely related to Mantisatta, despite the large physiological differences between them, and the similarity of those spiders with a group of genera they termed Marpissoida.

[7] In 2015, Maddison listed the genus within the tribe Ballini, derived from Simon's original name, but attributed to an earlier author, Nathan Banks from 1892.

[11] Pachyballus mombasensis is a wide-bodied spider that, like other members of the genus, resembles a beetle the family Chrysomelidae.

The spider's body is divided into two main parts: a round cephalothorax and a heart-shaped abdomen.

The chelicerae have three teeth to the back, while the remainder of the mouthparts, including are brown, except for small white edging to the labium and maxillae.

Its topside is dark brown while its underside is dominated by a scutum, which is similar to other species in the genus.

[13] This is very similar to Pachyballus flavipes, differing only in that the apophysis on this species hugs close to the cymbium.