Writing proficiency is a vital academic skill that must be developed among students; however, the correct use of language structure and adherence to writing standards remain a challenge for many. This study aims to examine the common errors committed by Senior High School students of the University of Mindanao in their academic essays. The researchers also aim to identify the types of errors with the highest and lowest frequencies, which will serve as a basis for future instructional interventions. The study employed Corder’s (1967) Error Analysis as the primary methodology in analyzing 100 academic essays written by students. Based on the analysis, substitution, or the selection of incorrect linguistic elements, emerged as the most frequent type of error. This was followed by errors in mechanics, addition (the insertion of unnecessary elements), omission (the absence of required elements), and finally, permutation (incorrect arrangement of sentence components), which had the least occurrences. The results also revealed that students struggle with the correct application of grammatical rules, such as the appropriate use of adjectives, proper affixation, sentence structure, and the accurate placement of elements within a sentence. The findings suggest that teachers must employ strategies focused on increasing grammatical awareness to further enhance students’ proficiency in Filipino and academic writing.