The current study compares simultaneous vs. successive bilingualism through a mixed-method research design with four four-year-old Korean-English bilingual children who were born and raised in the USA. Two simultaneous bilinguals were exposed to Korean and English from birth, whereas two successive bilinguals were exposed to Korean from birth, but to English between 18 and 24 months. Three direct assessment tasks (i.e. narrative, vocabulary, and syntax tasks) were used to assess children's multiple language domains in both Korean and English. A caregiver report supplemented direct testing. Findings indicate the simultaneous bilinguals often performed better in English than in Korean, whereas the successive bilinguals often scored higher in Korean than in English, but with some exceptions. For instance, one simultaneous bilingual demonstrated high Korean grammar performance even superior to successive bilinguals. Results suggest that simultaneous bilinguals do not necessarily differ from successive bilinguals solely due to the timing of language exposure; this could be due to varied language experiences. Furthermore, children's varying performances on each task indicate that bilinguals' language subskills develop differently, sometimes referred to as "bilingual profile effects." Findings have implications for defining simultaneous vs. successive bilingual acquisition, challenging views that age of exposure alone differentiates bilingual children significantly.