According to the obituary published in Locus Magazine, Edmondson was born in Rachauchitlán, Tabasco, Mexico.
Although generally called a science fiction writer he also wrote Westerns under the names Kelly P. Gast, J. B. Masterson, and Jack Logan. As he could also speak six languages he did translating work as well. His science fiction career began in 1955 with a story in Astounding. Over the years he produced several novels which gained some note for their interest in time travel and Latin America. Several writers, including Gardner Dozois, tend to see him as a neglected author.