If one has a job helping others (eg: assisting the mentally ill in finding employment) is one entitled to feel they are performing a Mitzvah while they work? Or only that they are performing their job?

A person who has the opportunity to have a profession that also allows the betterment of the human condition, certainly should thank G-d for this, and in that way, incorporate one’s work into the service of Hashem. Awareness that one is serving Hashem (i.e. doing a mitzvah) is fine as one as one doesn’t think he deserves a pat on the back. One also shouldn’t use it as an excuse not to do other good deeds.