Resistance is rare against the oomycete plant pathogen Phytophthora cinnamomi Rands. Only a limited number of species have been recorded as field-resistant species in Australia. However, understanding the nature of resistance of those species when grown under controlled conditions is challenging because of their slow growth and the inherent difficulties of working with a root pathogen. We assessed the Australian native species, Lomandra longifolia Labill., as a resistant species by analysing in detail the response of roots to infection by P. cinnamomi in a series of comparative tests with Lupinus angustifolius L., a highly susceptible species. Following inoculation of L. longifolia roots, lesion length and colonisation percentage were significantly less than in roots of the susceptible species. Moreover, there was no statistical difference in root growth rate, whole-plant FW and leaf relative chlorophyll content between controls and inoculated L. longifolia. We then examined three key cellular responses that are related to resistance: the production of the reactive oxygen species, H2O2, callose formation and lignin deposition in L. longifolia roots following inoculation with P. cinnamomi. The upregulation of these resistance-related components in the early hours after inoculation suggested their involvement in resistance and that this is controlled by the coordinated response of multiple components. Resistance assessment and a detailed investigation of cellular resistance components along with gene expression analysis provides a platform for further understanding of the mechanisms of resistance against this generalist pathogen and presents opportunities for manipulating susceptible species for disease resistance.