After the change of sovereignty from the United Kingdom to People’s Republic of China, Mandarin has become increasingly important in Hong Kong society. However, it is observed that many native Cantonese speakers have problems mastering Mandarin tones despite the less complex tonal system. This study attempts to investigate the Mandarin tonal error patterns made by Hong Kong learners and the possible causes for these errors. A reading aloud test and a follow-up interview were designed to collect the data. The findings of the study show that a large number of students were confused about Mandarin tones and found them difficult to master, especially between tone 1 and tone 4 and tone 2 and tone 3. All the participants made these two mistakes in the test. This study hypothesizes that L1 negative transfer is the key factor that helps Cantonese speaking learners’ ability to establish a new tone in the L2. However, the reason for participants’ tonal mistakes is not due to one single cause; instead, the errors are caused by multiple factors, including lack of tonal knowledge, incomplete tonal acquisition, and tonal errors explained by the Speech Learning Model. Copyright © 2021 John Benjamins Publishing Company.