Dr. Maurice J. Conley, 90, a dentist who practiced in Washington from 1919 until his retirement in 1978, died of cardiac arrest Tuesday at Suburban Hospital in Bethesda. He had cancer.

Dr. Conley was born in Somersworth, N.H., and grew up there. He graduated from St. Anselm's College in New Hampshire and then moved to Washington, where he earned his dental degree at Georgetown University in 1912.

For the next five years, he practiced in Biddeford, Me. He entered the Army in 1917 and saw World War I service in France. In 1919, he returned to Washington and established a dental practice. He remained active in it until his retirement two years ago. One of his grandchildren, Dr. Maurice J. Conley III, took it over.

Dr. Conley lived in Washington until about a year ago, when he became ill.

Since then he had lived with daughters in Arlington and Damascus. He recently had been in the Bethesda Retirement Center.

He was a life member of the D.C. Dental Society. He also was a member of the St. Thomas Apostle Catholic Church in Washington, the Knights of Columbus, the Cleveland Park Citizens Association and the Georgetown Alumni Association.

Dr. Conley's wife, the former Antoinette Ruppert, whom he married in 1917, died in 1963.

Survivors include four children, Mrs. Albert J. Orlosky of Arlington, Mrs. Charles E. Murray of Damascus, and Maurice J. Jr. and Robert O. Conley, both of Chevy chase; one sister, Mary A. Conley of Washington; two brothers, James J., of Washington, and the Rev. John J., of Richmond, Va.; 44 grandchildren, and 31 great-grandchildren.

The family suggests that expressions of sympathy be in the form of contributions of the American Cancer Society.