HPC companies hire many mathematicians. I work with several PhDs in applied mathematics. Another area is finance, as you mentioned. I interviewed at several he

HPC companies hire many mathematicians. I work with several PhDs in applied mathematics. Another area is finance, as you mentioned. I interviewed at several hedge funds and investment banks after my undergraduate degree (in physics and mathematics). Of course, it's not hard to go into CS for a masters or PhD which opens up many other options as well. This was more what I did. Almost all these positions require you to be capable of both whiteboard work, along with programming. So be sure you can develop software.