Research has shown tentative support for a “borrowed word effect” of English-based loanwords in Japanese (gairaigo) on written production in English by Japanese learners. This study interrogates a longitudinal learner corpus of argumentative and narrative writing by Japanese learners (NNS) and a corresponding NS corpus. Vocabulary profile analyses revealed: 1) NNS writing showed greater deployment of loanword cognate items than NS writing in both genres. 2) The deployment of loanword cognate items in NNS writing in both genres did not change over time. 3) NS writing showed greater deployment of loanword cognate items in narrative writing than argumentative writing, but NNS genres showed no difference. Keyword analysis and concordances of selected loanword cognate items revealed widespread and consistent patterns of ungrammaticality resembling L1 usage. Findings suggest Japanese writers heavily rely on loanword cognates. While loanword cognates arguably contribute to fluency, findings suggest potential for overreliance and negative transfer.