Never heard of him? Most people haven't. His fame as an Ivy League believer in alien abductions was more novelty than anything enduring but 'experiencers - alien abductees don't want to be called abductees - said he was “the most important scientist ever to dare to admit the truth about the abduction phenomenon.”
Which means they don't know what a scientist is. Nor did Oprah Winfrey, who unsurprisingly had him on her show. Eventually, Harvard didn't like looking like crackpots so he was subjected to an investigation - "ontological fascism", Mack called it. He was basically cleared. To prove aliens do not exist is nearly as hard as proving psychology does.
But the generation of social scientists who wanted things a little looser than what science would allow in the early part of the 20th century couldn't have anticipated Mack, who did a 'psychological study' of Lawrence of Arabia, was a devoted anti-nuclear protester and engaged in LSD "consciousness expansion". If a whole lot of people claimed to be abducted by aliens, he determined, it must be true. At least in a symptom-based field like psychiatry.
Now he is getting a motion picture made about him, courtesy of no less (and, really, no surprise) Robert Redford, who seems to be still trapped in 1987 in lots of ways.
Ralph Blumenthal at Vanity Fair has the whole story, in Alien Nation: Have Humans Been Abducted by Extraterrestrials?