For 30 years Mugridge was a specialist in American History in the General Reference and Bibliography Division of the Library of Congress and was a leading authority in his specialty and a widely known to scholars throughout the country. Mugridge was the author of a number of articles and book reviews on historical journals, an active member of the American Historical Association, the American Studies Association, the Columbia Historical Society and the Society of American Archivists.
He was educated in the Chicago public schools and received his B.A. and M.A. degrees from the University of Southern California and at the Harvard Graduate School where he continued his graduate work in history.
Before joining the Library of Congress in 1933, Mugridge served as a teaching fellow at the University of Southern California and as a research assistant to historian Samuel Eliot Morison, working on the Tercentennial History of Harvard University; he was also employed for a short time to do editorial work for the Colonial Society of Massachusetts and in 1942 he was out in charge of the Study Room Reference Service. After 1944 he served under several titles as Specialist in American History, except for a few months in 1946, when he was acting Assistant Chief of the General Reference and Bibliography Division, and for a brief period in 1949-50, when he was on leave from the Library to edit the Adams-Jefferson correspondence for the Institute of Early American History and Culture at Williamsburg, Virginia. Mugridge edited and contributed extensively to the monumental Guide to the Study if the United States of America, an annotated bibliography of representative books reflecting the development of American life and thought, which was published by the Library of Congress in 1960.
In recognition of the significant contribution to scholarship represented by his work on the Guide, the Library honored him in April of 1961 with a Superior Service Award. At the time of his death, he was directing the compilation of a supplement to the Guide.
Other noteworthy bibliographies edited or compiled by him included those on Christopher Columbus, American battle art authors published in America from 1892 to 1950 and the American Civil War.
He died in his home on Tuesday, November 3, 1964 of a coronary thrombosis. He left no immediate survivors.
On the fourth USCF rating list published in December 1951, Mugridge was rated at 2359. An October 1937 article on the mannerisms of master Chess Review had this to say about Mugridge: "Mugridge is a head-holder and chin-nurser par excellence. Being of a more restful nature than Winter, he does not seek to find out whether his head can be screwed on or off."