Born in Sussex, England, Billy Warren completed his education at St. Mary’s College prior to serving with the Imperial Yeomanry during the Boer War. He arrived in Banff in about 1903 after working as a clerk in London. Tom Wilson hired him as a packer but he soon demonstrated enough proficiency that after one season he became a guide, taking clients on short trips to build up his experience. He guided Mary Schaffer and Mollie Adams to Moraine Lake and this led to a long-term association with Mary, both professionally and personally. In 1905, Warren led these two ladies on a trip to the Ptarmigan Valley and the following year they ventured north as far as Wilcox Pass. Together with Sid Unwin, Billy guided their legendary trip in search of Chaba Imne (Maligne Lake) in 1907 and then the successful one to the lake and beyond in 1908. Mary Schaffer named Mount Warren in her guide’s honour during the exploration of the lake.
Billy continued in the guiding business, operating out of Field in Yoho National Park. Each year he took Mary Schaffer to a new area and their relationship evolved into marriage in 1915 when they settled in Banff. By 1919, Warren had left the outfitting business and became one of Banff’s leading businessmen, establishing the Cascade Garage and Banff Motor Company. The following year he acquired the Alberta Hotel and in 1921 founded Rocky Mountain Tours and Transport. He became one of Banff’s most prominent citizens.
Recalling the naming of Mount Warren in 1908, wrote, "Next rose a magnificent double-headed pile of rock, whose perpendicular cliffs reached almost to the shore...It was its massiveness, its simple dignity which appealed to us so strongly, and we named it Mount Warren, in honour of 'Chief,' through whose grit and determination we were able to behold this splendour." Regarding his skills as a guide she wrote, "There are older ones (guides), there are better hunters, perhaps, with wider experience in forest lore, more knowledge of the country, but for kindness, good nature (such a necessary adjunct), good judgment under unexpected stress, he had no superior."