These data comprise XRD mineralogical analysis of twelve soil and sediment samples from the catchment surrounding Lashmars Lagoon, Kangaroo Island (Karti/Karta), southern Australia. To assess possible changes of sediment sources to Lashmars Lagoon over the past 7 kyr that could impact the interpretation of the palaeoenvironmental history of the site, these data were compared to the downcore XRD data from the composite lake sediment core (see 'Chronology, µXRF, XRD and bulk organic geochemistry from a lake sediment core from Lashmars Lagoon, Kangaroo Island (Karti), southern Australia, for the last 7 kyr'). The catchment soil and sediment samples were collected during field campaigns in November and December 2021. Soil samples were taken of the different surrounding surface geologies and sediment samples were taken from the creek inflows and one outflow to the lake, as well as a mudline sample from the northern basin of the lake. Mineralogy was determined by semi-quantitative XRD analysis using a Bruker D8 ADVANCE Powder X-ray Diffractometer with a Cu-radiation source operating at 40 KV and 40 amps, scanning 2 theta from 5 to 65 degrees with sample rotation of 30 rotations per minute. Data were interpreted using the software package DIFFRAC.EVA (Bruker, Karlsruhe, Germany) and Crystallography Open Database (www.crystallography.net/cod/) reference patterns for identifying mineral phases. Semi-quantitative estimates of mineral composition were obtained by Rietveld modelling using the program DIFFRAC.TOPAS 4.2 (Bruker, Karlsruhe, Germany).