The note dated 1916 was discovered behind a fireplace in Kirkwall. Addressed to the serviceman's family in Wales, it was sealed and stamped but never posted.
Staff at Orkney Library wanted to identify the author and made a successful appeal on their blog.
The hunt reached Canada where a distant relative suggested the sailor may be David John Phillips from Llanelli. He contacted Mary Hodge in Chester who recognised the writer as her grandfather.