Is my work as a marketer in a company, whose owner's money might be illicit, making my earnings illicit as well?

The basic principle regarding what a person possesses is that it is his property. It is not required to investigate the source of people's money when dealing with them, and whoever does so risks falling into blameworthy extremism (tanattu') in Islamic law. Sheikh al-Islam Ibn Taymiyyah said: "The basic principle regarding what a Muslim possesses is that it is his property if he claims it is, or that he is its guardian." As long as your work in the company is permissible, there is no sin upon you in that or in what you earn, even if the company owner's money is mixed, for dealing with someone whose wealth is mixed is permissible, especially if the permissible portion predominates. And since you are not certain that his money is unlawful, the original principle, which is safety, remains.