Writing is a complex skill and one of the most difficult to master. A teacher’s weak writing skills may negatively influence their students. Therefore, reinforcing teacher education by first determining pre-service teachers’ writing weaknesses is imperative. This mixed-methods error analysis study aims to examine the cohesive errors in the writing of English as a Second Language (ESL) pre-service teachers of differing language proficiency levels—Medium and High-level, as indicated by the band levels achieved in the Malaysian University English Test (MUET). 200-word narrative essays were collected from 30 final-year ESL pre-service teachers from UKM via email. The study found that the Medium-level pre-service teachers made the most errors in lexical cohesion, reference and conjunction cohesion categories. However, High-level pre-service teachers made more errors in lexical cohesion, ellipsis and reference. Collocation proved the most difficult form of cohesion for both groups of pre-service teachers, while High-level pre-service teachers made more errors in ellipsis than the Medium-level pre-service teachers. Nevertheless, this still indicates that the pre-service teachers’ overall mastery of cohesive writing is insufficient. Therefore, the teaching of these cohesive devices should be further fortified in the linguistic courses undertaken by ESL pre-service teachers to ensure that they are well-equipped in all aspects of cohesive writing.