Studies that have employed the Linguistic Proximity Model to account for crosslinguistic influence in third language learning so far have used child simultaneous bilinguals as participants in their research designs. This study investigated adult sequential Kirundi-French bilinguals to uncover factors driving cross-linguistic influence in learning L3 English present perfect tense using the same Linguistic Proximity Model as a theoretical framework. To achieve that goal, ninety participants including thirty L1 Kirundi, thirty L1 French, and thirty L1 Kirundi-L2 French learners of English were recruited. Those participants were selected using a stratified random sampling technique that took into account their linguistic backgrounds and their scores on the proficiency measure. Data were elicited using a background questionnaire, a quick placement test, a grammaticality judgment task, and a translation task. Descriptive statistics, independent-samples Kruskal-Wallis tests, analysis of variance, and multiple comparisons served in the data analysis. The results showed that simultaneous facilitative and non-facilitative cross-linguistic influence from French and Kirundi were operational at both lower and advanced stages of L3 development, with lower-proficiency learners experiencing negative influence from L1 Kirundi during production. These findings further support the Linguistic Proximity Model as an underlying theory for cross-linguistic influence in third language learning. They can also serve as a guide in planning pedagogical activities for third language learners at differing stages of the target language development.