Hi Courtney,
It is 100% possible to live and work in Shanghai as a Mandarin-speaker. A few things to note:
- Mandarin has been the official language of the country for decades now, meaning that pretty much all business transactions (from the small purchase at a 7/11 to the larger business-to-business contracts) are negotiated and signed in Mandarin
- Shanghai is a very young city, with millions of people who live there having come from other parts of China or overseas, and it is highly unlikely they speak Shanghainese (or its related dialects) if they are not from the area
- If you do ever encounter someone who does not speak Mandarin fluently and only speaks to you in one of the dialects and you completely cannot understand them, chances are that they understand you and will adapt their speaking to match their understanding of Mandarin (which could be very low but still sufficient for brief conversations). I only encountered this situation once, and that was in Chengdu (not Shanghai, which is significantly more Mandarin-speaking than most cities)