Urban areas are increasingly recognized as important centers of biodiversity. Nonetheless, invasive species can reduce this biodiversity, and cities can be hubs for alien plant invasions, highlighting the need to monitor urban biodiversity and problematic alien species. The goal of our study was to assess the distribution of wild chervil [Anthriscus sylvestris (L.) Hoffm.] and anise [Myrrhis odorata (L.) Scop.] in green spaces of Reykjavík, Iceland. This information is necessary to implement the city's biodiversity strategy regarding invasive species. Both of these alien plants are spreading throughout Iceland, and Reykjavík's high-latitude location (≥63°N) and remoteness make it an ideal case study to assess alien plant introductions and invasions in subarctic urban areas. We surveyed four green spaces (Laugarnes, Vatnsmýri, Elliðaárdalur, and Ægisiða) from May to October 2017 using AllTrailsPro and ArcGIS mobile applications. ANOVA and Bonferroni correction (post hoc test) were used to compare the distribution and patch sizes of A. sylvestris and M. odorata among the study sites. We found that A. sylvestris covered at least 10% (15.5 ha) of the total area surveyed (158 ha), while M. odorata only covered ≤1 ha. Both plants were abundant near buildings, pathways, riversides, and streams, and they are expanding their distribution in Reykjavík's green spaces. While A. sylvestris is clearly more established and widespread with larger patches (>100 m²), the distribution of M. odorata is more localized, occurring mainly in smaller patches (<100 m²). We recommend long-term monitoring to further assess M. odorata's invasive potential, as well as testing and adopting integrated weed management strategies via adaptive management to control the distribution of A. sylvestris and that of other problematic alien plants. These actions, which are applicable to other subarctic cities, will help foster more proactive management encouraging urban biodiversity.