As a local, I would suggest that you stick to your Mandarin and do not be bothered with Cantonese (unless you would like to chat up with local girls...or you would like to understand the gossips among your colleagues). If you can speak English plus Mandarin, you can go anywhere in HongKong without troubles. All service industries speak the language. It is a fact that you may encounter some people on the street who cannot speak Mandarin, but you can always ask the next one. In particular, if you are a westerner speaking Mandarin, local people would be more interested. You can try it yourself and you will be amused!
Anyway, once you move in town, you will soon learn some Cantonese from TV/your colleague on day to day basis easily because you have Mandarin background. Cantonese is "funny" but extremely difficult to learn!