Mastery of English grammatical accuracy is paramount for developing proficiency as a foreign language. However, research examining tendencies among Saudi undergraduate writers remains limited. This study aimed to address such gaps through a comprehensive identification and categorization of errors present in student essays. A rigorous mixed-methods approach analyzed over 350 grammatical mistakes across 50 female students' writing samples. Errors were systematically coded and categorized to reveal frequency data through quantitative analysis. Findings demonstrated spelling, subject-verb agreement, and capitalization as the most prevalent issue areas, collectively accounting for over 75% of errors. Examples uncovered specific challenges involving verb tense, sentence fragments, and article usage. While outcomes aligned with previous literature, linguistic influences on persistent spelling difficulties were explored to broaden understandings. Recommendations for targeted exercises strengthening weaker domains like phonics rules and irregular verbs were provided. Continuing periodic error analyses can optimally inform evolving pedagogy tailored to emerging needs over time. Significantly, this novel diagnostic approach begins addressing critical literature gaps through data-driven insights that meaningfully inform curriculum development to better support the still under-researched Saudi EFL student population.