McKay, Scott and Whatmore claim that the first appearance of THE PEOPLE OF THE MIST was as a serialized novel, published by TIT-BITS WEEKLY from December 1893 through August 1894. I have never seen this serialization.
McKay, Scott and Whatmore also claim that Longmans, Green, and Company published the first edition in book-form in London on October 15, 1894. This edition, purportedly composed of 10,000 copies, contains 16 illustrations by Arthur Layard. Ads at the end of the book are dated-coded 9/94 (September 1894).
McKay and Scott also claim that Longmans published the first American edition in November 1894 with the same illustrations as its British edition. Ads at the end of the book are not dated.
George Newnes published its edition in London circa 1906, and this new edition features 8 illustrations by Cyrus Cuneo.
See the VisualHaggard.org page dedicated to the illustrations found in various editions.
Longmans' Colonial Library reprint edition
Excellent-condition Longmans reprint with original DJ (not in my collection)