The son of William A. and L. Fern Smith Armstrong was born in Indianapolis, Indiana on June 13, 1914. He grew up in the nearby community of Shelbyville where he was graduated from Shelbyville High School in 1932. He played the saxophone in the high school band and his record in pole vault stood for many years.
He attended evening classes at Indiana University (1937–45) and Butler University (1945–46) but he did not complete the requirements for a degree. Honorary Doctor of Laws degrees were bestowed upon him in 1975 by Ball State University and Indiana University, where he lectured in accounting for ten years (1945–55). In 1985 he received an honorary Doctor of Business Administration degree from Butler University, where he served as a visiting professor of accounting (1978–82). During his professional career he maintained his association with Indiana University, serving as president (1954–55) of its Executive Development Program and a member of the Advisory Committee of the University's Management Institute (1956–57), and the Dean's Advisory Board of the School of Business (July 1974-July 1977). In 1974 he was elected as a member of the Indiana University Academy of Alumni Fellows. He was certified as a CPA in 1946 (Indiana).
His first job (1933) was as a vacuum sweeper salesman; he sold only one in an entire year, and that to a customer who was unable to pay. This was his first experience with an allowance for a bad debt. Over the next several years he held various jobs including shoe salesman for one year (1934), insurance agent (1935–36), manager of a shoe store for two months, and janitor of a utility company (1937). From the latter position he moved to the utility company's accounting department in 1938. In 1942 he left the field of utility accounting to become a staff accountant with the accounting firm Geo. S. Olive & Co. He became a partner in 1947 and managing partner in 1970, a position he held until he resigned to become the first chairman of the FASB, effective November 1, 1972. He started the job from a desk in the AICPA offices. He was one of the founders of the Financial Accounting Foundation (the parent of the FASB) and when he left the office of chairman on December 31, 1977, he left behind an effective organization of over 100 staff members. He is Chairman Emeritus of the FASB, and during 1978-79 he served as advisor to the Financial Accounting Foundation.
He was active in the AICPA, serving as president (1970–71), vice president (1969–70), and member of the Board of Directors (1970–72); he is a lifetime member of Council. As president of the AICPA, he appointed the Wheat and Trueblood committees. The Wheat Committee recommended the formation of the FASB and the Trueblood Committee emphasized the need for a reporting framework. He has also been on a number of AICPA committees including membership on the Committee on Auditing Procedure (1957–62), and Committee on Practice Review (1962–63). He was chairman of the Project Advisory Committee for the accounting research study on corporate stockholders' equity and the Committee to Study the Displacement of Local and Regional Firms Involved in SEC Registration (1962). For the years 1963-69, he served on the Accounting Principles Board. In 1960 he helped form the American Group of CPA Firms, a private national federation of regional firms; he served as its first chairman of the Board of Governors (1960–63). He served the Indiana Association of CPAs in numerous capacities including vice president (1960–61), president (1961–62), member of its Board of Directors (1960–62), and trustee of its Educational Foundation.
He spoke before many professional groups and he has written a number of articles for professional journals. He was a contributor to the Handbook of Accounting and Auditing published in 1981.
He was selected as an honorary member of Beta Alpha Psi and Beta Gamma Sigma. He was the recipient of the 1973 New York City Beta Gamma Sigma honor award and in 1975 he received the Distinguished Service Award from the University of Hartford's Center for the Study of Professional Accounting. His allegiance to the profession was again recognized in 1977 when he was awarded the AICPA's Gold Medal Award.
His civic and community activities are numerous including membership on the Advisory Board of St. Francis Hospital in Indianapolis (1969–72), Board of Overseers of The Wharton School of the University of Pennsylvania (1973–81), Advisory Board of the Securities Regulation Institute of the University of California (1974), and Board of Directors of J. C. Penney Company, Inc. (1978–85) and Nixon Enterprises, Inc. Marshall's private enjoyments included golf, fishing, boating, playing with his grandchildren, and music. In his youth, he sought a career as a professional musician and did, for a while, play the saxophone professionally. Mr. Armstrong married Marjorie L. Thompson on September 24, 1939. She preceded him in death, on May 28, 2000. Together, they had three children, the late Julie Armstrong Alexander, surviving are David Armstrong and his wife, Linda, of Aurora, Indiana and Kristi Armstrong Clerkin of Carmel, Indiana. His beloved grandchildren include David Alexander, Kelli Alexander Locke, Mark Clerkin, Alison Clerkin Laitas and her husband, Tim, Cutler Armstrong, Collin Armstrong, great grandchildren, Kelsey Alexander, Julianne Locke and Andrew Laitas.
Armstrong died surrounded by his family at St. Vincent Hospice in Indianapolis on October 23, 2005, aged 91.