The question of whether bilingual language experience confers a cognitive advantage remains open. Controversy arises from assertions that putative advantages can instead be explained by differences in culture, socioeconomic class, or immigration status, as well as the classification of bilingual experience as a fixed variable rather than a random effect. The present study addresses these issues by assessing the impact of variability in English (L2) language experience on executive function in a group of Mandarin-English speakers (n = 41) from Shenzhen. Participants reported on demographic details, language history, perceived stress, and performed a Simon task online. Data were analysed using Linear Mixed Effects (LME) models to test for individual differences on Simon task performance. Results showed higher levels of L2 proficiency were associated with reduced Simon effects, suggesting a cognitive advantage.