M. Marcus Klein studied medicine in Sweden and then qualified as a medical practitioner by passing the Staats Examen in Berlin, Germany, in 1893. His academic qualifications included both the degrees Doctor of Medicine (MD) and Doctor of Philosophy (PhD). In 1896 he was admitted to practice in the South African Republic (Transvaal) and settled in Christiana to do so. However, by 1897 he had moved to Klerksdorp. After the Anglo-Boer War (1899-1902) he published a paper on "Dietary care of native mine labourers" in the first volume of the South African Medical Record (1903/4). In addition to his medical practice he was the owner of several prospecting claims on the farm Driefontein No. 148. By 1907 he was practising in Brakpan. The next year he was appointed justice of the peace at Boksburg, where he remained until his death. Also in 1908 he read a paper before the Transvaal Medical Society on "Kaffir beer; its manufacture on the Rand and the need for adequate supervision", dealing with how it was brewed, its chemistry, storage, fermentation, and the need to control its manufacture. The paper was published in the Transvaal Medical Journal (1908, Vol. 4, pp. 83-90).
Klein was a naturalized British subject and was not married.