GHOST is a high resolution spectrograph system currently being built for the Gemini South Observatory in Chile. In the Cassegrain unit, the observational targets are acquired on integral field units and guided during science exposures, feeding the fiber cable to the temperature-stabilized echelle spectrograph. The Cassegrain unit is mounted on the Gemini telescope, and consists of a main structural plate, the two object positioners and ballast frame. The image from each of the two science beams passes through a field lens and a mini-atmospheric dispersion corrector and is then captured by the integral field unit. The positioner moves each corrector-integral field unit assembly across the focal surface of the telescope. The main structural plate provides the interface for the positioner and ballast frame to the telescope structure. In this paper we describe the final design and assembly of the GHOST Cassegrain unit and report on the outcome of on-sky testing at the telescope in Chile.