Although English may officially be Malaysia’s second language, it is certainly not true for all. A case in point is that of the KadazanDusun people of North Borneo. The Kadazans and the Dusuns, the largest single language community in the state accounting for 24% of the population. They speak a standardized form of their language, KadazanDusun, which comprises a number of dialects understood by all. After Independence and integration with Malaya in 1963, the emphasis was placed on the adoption of the Malay language. The consequence of this is that Malay is now typically the second language (L2) of the KadazanDusun speaking community, while English is relegated to a third language. Having more than one language to contend with can create problems for ESL learners. Knowledge of the first language is often applied incorrectly to the target language, leading to errors and confusion over meaning. Relatively little research has examined the errors made by KadazanDusun ESL learners and equally little is known about the impact of the mother tongue on their written English. Identifying and analyzing the source of these errors should enable teachers to tailor education programmes and teaching materials so that they address the specific needs of KadazanDusun ESL learners. This research aims to examine relationships between the observable L1-induced errors and English essay writing. In addition, this exploratory effort seeks to identify significant correlations, if any, between the various types of errors made. Since the present study is a contrastive study of errors in the English writing of learners, a quantitative method was considered appropriate for this research. The association between mother tongue related errors and English essay writing can be determined through descriptive statistics, reliability analysis and correlative analysis. In the present study, reliability coefficient for the research instruments used in the study has been included and the Cronbach’s alpha for the data is 0.76. .The mean values for all constructs range between 0.056 and 3.482. The findings have found some transfer effect was evident in the students’ essays. In other words, it is likely that the students’ L1 has some influence on their English writing, resulting in the errors found. Negative transfer appeared to be more significant than positive transfer in the students’ essays. A total of 31 grammatical errors were detected and 8 of these were identified as being probably influenced by the learners’ L1. The most frequently occurring errors made by KadazanDusun ESL learners were those with the highest mean Likert values, such as those involving singular / plural nouns and unusual sentence structures, while those with the lowest mean values, such as ellipses and present continuous, were the least frequently occurring. In terms of correlations in the writings of the learners, mother tongue-induced adjective errors typically include errors in spelling, subject-verb agreement, singular-plural nouns, adverbs, unnecessary articles and modals. Since further research is necessary to provide concrete evidence and conclusive findings, the current research acts as a leap pad for more challenging and interesting research in the future.