Secondary sources were giving me contradictory indications on this, so I went looking for primary sources. The closest thing I could find to a denial of mentalism was this by John B. Watson:

With the advent of behaviorism in 1913 the mind-body problem disappeared — not because ostrich-like its devotees hid their heads in the sand but because they would take no account of phenomena which they could not observe. The behaviorist finds no mind in his laboratory — sees it nowhere in his subjects. Would he not be unscientific if he lingered by the wayside and idly speculated upon it; just as unscientific as the biologists would be if they lingered over the contemplation of entelechies, engrams and the like. Their world and the world of the behaviorist are filled with facts — with data which can be accumulated and verified by observation — with phenomena which can be predicted and controlled.

If the behaviorists are right in their contention that there is no observable mind-body problem and no observable separate entity called mind — then there can be no such thing as consciousness and its subdivision. Freud's concept borrowed from somatic pathology breaks down. There can be no festering spot in the substratum of the mind — in the unconscious — because there is no mind.

-- John B. Watson, The Unconscious of the Behaviorist

Later on in the same document, when Watson says...

May I stop for just one moment to give the behaviorist's view of what most people mean or at least should mean when they say they are conscious or have consciousness? They mean, in my opinion, that they can carry on some kind of brief sub-vocal talk with "themselves" behind the closed doors of the lips.

...he would seem to be referring to actual small movements of the tongue, according to his earlier work Is Thinking Merely the Action of Language Mechanisms?

I had read this earlier work and thought that, while wrong in its reduction, it was not outright eliminative with respect to thinking - thinking was to be explained in terms of subvocal tongue movements(!) but still explained. However, I get the impression that Watson grew more extreme over time (a common phenomenon).

I am unable to find online the referenced text of his 1930 attack on the mind as an "old wives' tale".

But I don't think that "major behaviorists denied the existence of the mind" is straw.