While Karl Marx is considered one of the "Big Three" classical sociological theorists alongside Max Weber and Emile Durkheim, he would not have considered himself a sociologist and we don't even know if he was ever exposed to sociology to any meaningful degree, which would have been in its very nascent stages by the time of his death. Apocryphal legend has it that he dismissed sociology as a "silly bourgeois science" but nobody knows if that's true.

Also, sociology as an institution in and of itself was fairly nebulous--it was more of an approach than it was a discipline--until Durkheim founded the first sociology department at the University of Paris. Weber was considered an "economic historian" who did sociological work. Comte is regarded as the person who coined the term sociology but he's kind of like the crazy ancestor the family doesn't want to fully embrace, and his idea was more along the lines of outright social engineering than an academic discipline that researches social phenomena. You had Herbert Spencer, but he was more of an outlier--more of a popular lecturer than a university professor--and Marx would have despised his ideas (I think he did, in fact). Ironically, they're buried very close to one another in Hyde Park.

As for Marx, he would have regarded himself as a philosopher following on the heels of Hegel, Feuerbach, Kant, etc. That is, when he wasn't doing journalism to (barely) pay the bills. Today, sociologists and philosophers are as distinct from one another as any other two departments (hard sciences and occupations aside), despite what the more theoretically inclined among the former might like to think.