Which crusader for the moral and humane treatment of mentally ill individuals campaigned tirelessly in an effort that came to be known as the mental hygiene movement?

Dorothea Dix.

Explanation:

Dorothea Dix's main idea was to develop a humane psychological treatment for the individual, for anyone regardless of their lifestyle.

Moral therapy began to develop as a natural and respectful treatment for patients, centering on the moral and humane treatment for the mentally ill.

Dix began to develop the mental hygiene movement after attending a jail in Massachusetts in which she was witness to the deplorable conditions the patients faced, thus becoming a crusader for a humane treatment.