Dr. Mordecai Wyatt Johnson Jr., 59, a dentist who maintained a private practice in Washington for more than 25 years before retiring in 1970 and was a dental officer with the Department of Human Resources' R Street Clinic at the time of his death, died Saturday during open heart surgery at George Washington University Hospital.

Dr. Johnson was born in Cambridge, Mass. He was a graduate of Howard University, and a 1945 graduate of its dental school. His father, the late Mordecai Wyatt Johnson, was a past president of the university.

Dr. Johnson practiced dentistry in Texas and taught physiology and anatomy at Praire View A&M College there for three years before returning to Washington in 1948. During the next three years he maintained a private practice and worked as a dentist with the Group Health Corp. here. He served in the Army during the Korean War. He had been a dentist with the city for the past 19 years.

He was a member of the American Dental Association, the Howard University Dental Alumni Association, and the Howard University Alumni Association of Washington. His hobbies included golfing, and he was a member of the Pro Duffers Golf Club. He was president of the What Good Are We Club of Washington.

Survivors include his wife of 30 years, Enid Tucker Johnson, and three daughters, Carla Ann Buczinski, Madelyn Elise Johnson, and Diana Ethelyn Johnson, all of Washington; two other daughters, Carmen Williams of Oxen Hill, and Jo Louise Alston of New Rochelle, N.Y.; two brothers, Dr. Archer Johnson of Puerto Rico, and William Howard Johnson of Washington; two sisters, Carolyn J. Graves of Philadelphia, and Anna Faith J. Jones of Newton, Mass., and four grandchildren.

The family suggests that expressions of sympathy be in the form of contributions to the Mordecai Wyatt Johnson Memorial Scholarship Fund of Howard University.