Already in 1951 we find a Vienna Circle positivist (and physicist-philosopher) reacting against claims about 'scientism'. Philipp Frank wrote:
All of these groups have attempted to limit science to the collection of material and to enforce a 'philosophical interpretation' which supports the 'way of life' which they regard as a 'good' one. For this reason, they will insist upon an autonomous philosophy that will provide decisions where science, in the narrower sense, has reached a stalemate. Organizations like the Roman Church or the Third Reich in Germany [yes, he just did that] have denounced in the strongest possible terms the claims of 'active positivism' or, as Maritain puts it, of 'scientism' to discuss the high-level generalizations of science in scientific terms. We find the same attitude toward 'active positivism' in the announcements of the Communist Party in the Soviet Union. They require positively a philosophy which is to control scientific work. (Frank, 'The Logical and Sociological Aspects of Science', Proceedings of the American Academy of Arts and Sciences 80 : at 28-29)In his Ransoming the Time (Charles Scribner's Sons, 1941), Jacques Maritain had written about 'the pseudo-metaphysics of scientism' (p. 54). Maritain had been condemning scientism in his French works as far back as 1910 (in 'La science moderne et la raison', Revue de philosophie) though I don't think he used the term 'scientisme' that early. According to James Schmidt, Maritain used 'scientisme' in books published in 1927 and 1936.
Étienne Gilson's Elements of Christian Philosophy (Doubleday, 1960) has a section titled 'Materialistic Scientism', a view that, he says, is exemplified by Marxism (p. 94).
Update (Dec. 21): Frank's rhetorical strategy is interesting. Against the Cold War tendency to place scientism in the same camp as Marxism, Frank places scientism (or active positivism) in opposition to this ideology as well as to traditionalist and far right outlooks. His reply is similar to that of Seymour Martin Lipset (quoted by Tommi Uschanov in a comment on my 4th post in this series). It's also interesting that Frank doesn't actually speak in terms of ideologies; instead, he confines himself to claims about the observed behaviour of identified 'groups' and 'organizations'.