He received his early education in Baltimore schools and at the age of fifteen entered the University of North Carolina. In college he majored in chemistry, working under Charles Baskerville, and after graduating, B. S. , in 1904 he went with Baskerville to teach chemistry at the City College of New York. Soon afterwards, however, he became an assistant to Dr. C. G. L. Wolf in the department of physiological chemistry of the Cornell University Medical School in New York, where he spent the next six years learning the fundamentals of biochemistry, assisting in the laboratory instruction, and, rather incidentally, attending enough classes to obtain his M. D. degree in 1910.
Marriott's real interest being in biochemistry, he took no further hospital or clinical training. In the fall of 1910 Marriott went to St. Louis as instructor in the department of biochemistry in the newly reorganized Washington University Medical School. After four years, however, wishing to apply his biochemical knowledge to the direct study of clinical problems and having been unable to find a position in the field of his choice, internal medicine, he accepted an offer to become an assistant to Dr. John Howland in the department of pediatrics at the Johns Hopkins Medical School. Marriott's work with Howland marked the start of a brilliant career in pediatrics. Howland was an able clinician but, like most clinicians of his generation, untrained in the basic disciplines necessary for scientific investigation. Applying his biochemical knowledge to Howland's clinical problems, Marriott devised the technical methods that led to their joint work on acidosis in infancy, which received immediate international recognition. Another of their important contributions was the discovery of the low serum calcium concentration in tetany. On the strength of this work the Washington University Medical School offered Marriott its professorship of pediatrics in 1917, despite his limited clinical experience and training. Under his leadership the department developed rapidly and attracted men from all over the country and abroad, a half dozen of his assistants later becoming professors of pediatrics in other medical schools. Marriott's most important scientific work at St. Louis centered on the toxic condition in infancy known as "alimentary intoxication. " The cause was controversial and had been the subject of much study, particularly in the European clinics. Marriott showed that it was not a matter of food poisoning, as some considered, or of infection, but a group of symptoms brought about by a reduction in blood volume due to water loss. He gave the name "anhydremia" to the syndrome, and the discovery of the pathogenesis of the condition led to the development of therapeutic measures for its prevention and treatment. Another important contribution was in the field of infant feeding. For years, following the teaching of Thomas Morgan Rotch of Boston and Luther Emmett Holt of New York, infant feeding in the United States had been dominated by the so-called "percentage method. " This was a complicated system involving mathematical formulas and was exceedingly cumbersome as well as scientifically unsound. Marriott, uninhibited by tradition because of his lack of clinical training and experience, approached the subject from the viewpoint of a biochemist and introduced simplified procedures, based on inexpensive evaporated milk and corn syrup in place of earlier, more costly ingredients. The "Marriott method, " as it was often called, came into general use among practitioners and was an important factor in placing the artificial feeding of the infant on a sound common-sense basis. In 1930 Marriott published his Infant Nutrition, which went through three subsequent editions. In 1923 Marriott became dean of the Washington University Medical School. He found administrative duties to his liking, and after 1923 his research interests waned. His later clinical papers were not of the same caliber as his earlier scientific work; some, indeed, were severely criticized and proved to be in error. These mistakes, however, which reflect Marriott's scant clinical training, are of minor importance when compared to his significant contributions in the field of metabolism and the metabolic diseases of infancy and childhood. In 1936 Marriott left Washington University to become dean and professor of research medicine at the University of California Medical School in San Francisco. He had scarcely entered on his duties there when he developed a fatal streptococcic infection complicating an abdominal condition following appendicitis. He died in San Francisco before the end of the year, still only fifty-one, and was buried in Greenmount Cemetery in Baltimore.
A large and active man, Marriott lived for his work and his family. He had a remarkable enthusiasm for whatever he was doing that was transmitted to his associates. He was rather shy, except with intimate friends; this shyness was sometimes mistaken for conceit, as was his steady confidence in himself, his work, and his plans. He was an excellent teacher, with an unusual facility for presenting complicated problems of metabolism in a clear, understandable way.
In 1911 Marriott married Elizabeth Devereaux Robinson, daughter of John Beverley Robinson, professor of architecture at Washington University. They had twins, Elizabeth Devereaux and McKim.