More than a decade after John N. Mitchell stepped down as attorney general to run President Richard M. Nixon's reelection campaign, the Justice Department still lacked a portrait of its former boss. This Friday, however, Mitchell finally will join the ranks of other former attorney generals when his portrait is hung formally at the department.

The painting's artist, Gloria Schumann of Muskogee, Okla., said the portrait shows Mitchell's "strength, his courage and his integrity." She attributes those qualities to Mitchell based on her long acquaintance with him and the fact that he "never turned around and blamed anyone" for his conviction on charges of conspiracy, obstruction of justice and making false statements to a grand jury in the Watergate scandals. Mitchell served 19 months in prison and was released on parole in 1979.

Schumann, who painted an inaugural portrait for Nixon in 1969, received $15,000 from the Justice Department for Mitchell's portrait.

She said she met the Mitchells in 1964, when she painted a portrait of their daughter, Marty, who was then 4. Schumann lived then in Harrison, N.Y., near the Mitchells' home in Rye. In 1970 and 1972, she painted portraits of Martha and John Mitchell for their home.

Mitchell asked Schumann to paint the portrait last February. "He asked me if he had to come out for a sitting and I said no," Schumann said, adding that she referred instead to materials she had collected during Mitchell's previous sitting for her, including photographs and notes on his features.

Schumann said she believes that this painting is better than the previous one because she has more experience and because her conversations with Mitchell over the past year made her realize that "there's much more to the man than last time. We're both 15 years older."

Justice spokesman Thomas P. DeCair said the portrait will be hung in a hallway on the second floor of the agency.

Mitchell, 71, a partner in Global Research International Inc. here, could not be reached for comment. COMINGS AND GOINGS . . .

Ronald L. Blunt, special assistant to the attorney general since September 1983, has been appointed counselor to the attorney general by William French Smith. Blunt graduated from law school in 1981 and then served as a clerk to Supreme Court Justice William H. Rehnquist.

James M. Spears has been appointed acting assistant attorney general in charge of the Office of Legal Policy. Spears previously served as a deputy assistant attorney general for the Land and Natural Resources Division.