Marcus was born on February 5, 1875, at Rock Island Arsenal, the son of Captain Marcus Ward and Lydia Anne Lyon. In his early years, he lived at various army stations where he became interested in nature and started to collect. He went to Rock Island High School and graduated in 1893, and in 1897 he received a degree from Brown University.
After graduating from college, Marcus worked for one year as a bacteriologist in North Carolina Medical College. In 1898, he moved to Washington, D.C., and worked as an aid and as an assistant curator in the Division of Mammals of the U.S. National Museum of Natural History. He went to Venezuela in 1899 with Lieutenant Wirt Robinson to collect mammals for the museum. Marcus attended George Washington University, earning his MS degree in 1900, his MD degree in 1902, and his PhD degree in 1913. During this period he also was teaching physiology and bacteriology in the Medical School of Howard University. He married Dr. Martha Maria Brewer of Lanham, Maryland, in 1902. In 1904, Marcus traveled to the Louisiana Purchase Exposition in St. Louis and in 1905 he attended the Louis and Clark Exposition in Portland, Oregon, on both trips representing the U.S. National Museum. Then in 1911, he and his wife toured Europe, visiting museums and zoologists. From 1915 to 1918, he taught for the Medical School of George Washington University on the subjects of bacteriology, pathology, veterinary zoology, and parasitology. In 1919, he and his wife both were hired at South Bend Clinic, where they worked for many years. During World War I he worked as a pathologist in Walter Reed General Hospital and in 1919 was appointed to be a major in the Medical Reserve Corps.
Marcus wrote over 160 papers, focusing on mammalogy and pathology. He also was an active member in scientific organizations. He was corresponding secretary in 1904 and recording secretary from 1915 to 1919 of the Biological Society of Washington, treasurer and then president of the Indiana Academy of Science, and president of the American Society of Mammalogists from 1931 to 1933.
Marcus was elected as a member of the Washington Biologists’ Field Club in 1917. He died in 1942.