The observation of informalities in academic writing has become tremendous in recent years, as revealed by a plethora of cross-cultural and diachronic studies. Different users of English have already been explored; however, none has centered on the case of English as a Second Language (henceforth ESL) writers, such as Filipino academic writing scholars, in terms of delineating informalities in their academic writing discourse over time. For this reason, this quantitative study aimed to diachronically analyze informality features in academic writing of Filipino ESL researchers in the applied linguistics field. Using the framework of Hyland and Jiang (2017), a significant association between the years of writing and overall use of informalities was found, signifying an increasing number of informality features as time progresses. Specifically, there were five informality features that the writers noticeably observed over the years. However, only two categories of the informality framework, unattended anaphoric pronouns, and sentence-initial conjunctions or conjunctive adverbs, were most pronounced in writing and were increasing. On the other hand, the categories of first-person pronouns, sentence-final prepositions, and listing expressions were less salient and were decreasing. This paper ends with pedagogical implications for academic writing instructions as far as appropriating such features as (in)formal is concerned.