This study investigates developmental changes in grammatical subjects in the English writing of Japanese learners, focusing on subject position as an index of syntactic growth. Previous L2 assessment research has emphasized clausal subordination and global complexity indices; noun phrase complexity has attracted growing attention, but the subject position itself has rarely been isolated as a unit of analysis. Drawing on a longitudinal corpus of 26,399 texts written by 1,086 Japanese high school students over three years, subjects were automatically extracted and annotated using SubjEx, a dependency-parsing system developed for this purpose (accuracy: 93.5%). Mean subject length increased steadily across grades and CEFR proficiency bands, with clearer differentiation from B1 onward; a mixed-effects model with a random slope further indicates increasing divergence in learners’ growth trajectories. Subjects shifted from pronoun-dominant forms toward more complex noun phrases, with greater use of adjectival modification, possessives, compound nouns, and prepositional phrases, while lexical analyses showed a decline in first-person subjects and a rise in informationally dense nominal patterns. Subject position thus emerges as a sensitive indicator of L2 writing development, reflecting a shift from personal expression toward condensed informational packaging, with implications for complexity research and proficiency assessment.