John E. Clennell, son of Walter Cash Clennell and his wife Charlotte Constance, born Fraser, was a metallurgical chemist. In December 1895 his paper, "Notes on the analysis of cyanide solutions", was read before the Chemical and Metallurgical Society of South Africa and subsequently published in the society's Proceedings (Vol. 1, pp. 203-211; comments on the paper, pp. 218-221). A second paper by him, "Notes on experimental metallurgy" was received from the United States and read before the society on 17 December 1898. He was still an associate of the society in September 1899, but no longer after the Anglo-Boer War (1899-1902). Three other papers by him, published in the Journal of Physical Science, all three in 1895, were titled "On the estimation of alkaline hydrates and carbonates in the presence of alkaline cyanides", "On the estimation of cyanogen in impure solutions...", and "On the estimation of simple cyanides in the presence of compound cyanides". A few years later he wrote "The estimation of copper in cyanide solutions" for the Journal of the Society of Chemical Industry (1900).
Clennell subsequently wrote several important works on the cyanide treatment of ores, which were published in New York: The chemistry of cyanide solutions resulting from the treatment of ores (1904, 2nd ed. 1910); The cyanide handbook (1910, 520p; 2nd ed. 1915); and Cyanide precipitants, a collection of articles from the Engineering and Mining Journal (1914). He was also co-author of a book on metallurgical analysis, published in New York in 1930.