The name Kathlyn is a variant spelling of the English name Cathleen, which is derived from the Irish Gaelic name Caitlín. Caitlín is a diminutive form of the ancient Irish name Cáit, which comes from the Old Irish cáed meaning "pure."

The name Cáit has been used in Ireland since the early medieval period. One of the earliest recorded instances of the name is Saint Cáit, a 6th-century Irish princess and nun who founded a monastery in Inishcaltra. The spelling Kathlyn began appearing in the 19th century, likely influenced by the growing popularity of names with a "lyn" ending.

One of the earliest known bearers of the name Kathlyn was Kathlyn Williams (1879-1960), an American silent film actress who starred in several serials and feature films in the 1910s and 1920s. Another early Kathlyn was Kathlyn Hilliard (1900-1948), an American actress and dancer who appeared in Broadway musicals and Hollywood films in the 1920s and 1930s.

In the literary world, Kathlyn Michaels (1943-2012) was an American writer and editor who published several novels and short story collections. Kathlyn Gillette Hendricks (born 1945) is an American writer, teacher, and co-author of several books on relationships and conscious living.

In the field of science, Kathlyn Cooney (1922-2015) was an American chemist and educator who made significant contributions to the study of carbohydrate chemistry and mentored numerous students during her long career at the University of California, Berkeley.

Kathlyn is a name with deep roots in Irish history and culture, dating back to the early medieval period. Its variant spelling with the "lyn" ending emerged more recently, but the name has been borne by notable figures across various fields throughout the 20th century.