The fragmentation of natural habitats by urbanization often results in small and isolated remnants that lead to the degeneration of ecosystem services (ESs) and a loss of biodiversity. However, our understanding of the effects of landscape patterns on the provisioning of ESs in urban areas remains limited, as few studies have distinguished historical forest remnants (HFRs) from recently established habitats or have quantified changes in ESs due to HFRs. This study measured changes in the extent of forest ecosystems and the role played by HFRs in generating ESs in the human-dominated modernized landscape of Seoul, Republic of Korea. We use the spatial configuration of habitats to measure an index of habitat quality as an ES for a target conservation objective of general terrestrial biodiversity. Land-cover maps from two periods were used to identify 37 isolated patches within current parks with HFRs. Then landscape patterns and modeled habitat quality and habitat units (HUs) were quantified. Pearson correlation coefficients and forward stepwise multiple regression were used to examine the landscape metrics combined with HUs to explore key indicators that affect the HUs-value of HFRs. A 35.31% decline in total HUs was observed due to a significant decline in total forest area; however, the HUs of the HFRs increased by 0.5%. The shapes of current forests may positively affect the biodiversity of HFRs, whereas the area of newly formed habitats may negatively affect biodiversity in our modeling results. Thus, the careful design of newly formed habitats during city planning should include the preservation of historical remnants.