The term "Alienist" was heading for obsolescence until it was returned to a position of some prominence by the 1994 book "The Alienist" by the historian/novelist Caleb Carr.
The story of "The Alienist" starts In New York City in March 1896 when reporter John Schuyler Moore is called to the East River by his old Harvard classmate and friend Dr. Laszlo Kreizler, a psychologist, or "alienist", to view the mutilated body of an adolescent boy. The reporter and the alienist are enlisted as a team by the new police commissioner (who happened to be Teddy Roosevelt) to look into the murder. An "alienist" is "one who treats mental diseases; a mental pathologist; a 'mad doctor,'" according to The Oxford English Dictionary (OED).
The OED also defines "alienation" as in this sense as "mental alienation; withdrawal, loss, or derangement of mental faculties; insanity." The insane were thought estranged (alienated) from their normal faculties. The root of "alienist" is the Latin "alienare," to make strange. The word "alienist" came across the Channel to England from France where "aliene" meant insane and an "alieniste" was one who cared for the mentally ill: a psychiatrist.