Born in Butte, Montana on May 31, 1879 (most sources incorrectly list 1888) of Norwegian and Welsh heritage, Kathlyn was born to Joseph E. and Mary C. Williams. With early interest and experience as a vocal recitalist, she eventually attended the Sargent School of Acting and later studied at Wesleyan University (1899). Following stage experience in local stock and touring companies (from 1902) she began to develop a solid name for herself in such plays as "When We Were Twenty One". Her early career was generously sponsored by Senator W.A. Clarke after Kathlyn's family lost their fortunes. She eventually went to Hollywood while performing with the Belasco Stock Company and began making films as early as 1908 with D.W. Griffith at the Biograph Studio.
A popular star at the Selig Polyscope Company in 1910 (she was at first publicized as "The Selig Girl"), she appeared in assorted jungle adventures for the studio as well as a number of westerns opposite cowboy star Tom Mix. She made history, however, with the very first serial adventure, which contained a number of wild animals, and saved the faltering studio from bankruptcy. She proceeded to remain a popular item after being handed the lead in the Selig epic version of The Spoilers (1914) playing her signature role of Cherry Marlotte.
Badly injured in a car accident in 1949 in which she lost her right leg, Kathlyn became a wheelchair-bound invalid in the last decade of her life. She died in her Hollywood apartment of a massive heart attack on September 23, 1960, at age 81, and her ashes interred in the Deodora Hall, South Columbarium in the Chapel of the Pines Crematory in Los Angeles.