This study examined the linguistic and individual-level factors that render case marking a vulnerable domain in English-dominant Greek heritage children. We also investigated whether heritage language (HL) children can use case-marking cues to interpret (non-)canonical sentences in Greek similarly to their monolingual peers. A group of six- to twelve-year-old Greek heritage children in New York City and a control group of age-matched monolingual children living in Greece participated in a production and a picture verification task targeting case marking and (non-)canonical word order in Greek. HL children produced syncretic inflectional errors, also found in preschool monolingual children. In the comprehension task, HL children showed variable performance on the non-canonical OVS but ceiling performance on the SVO conditions, which suggests influence from English. Linguistic factors such as case transparency affected comprehension, whereas child-level factors such as proficiency and degree of (early) use of Greek influenced performance on both modalities.