Prince George's County Executive Winfield M. Kelly Jr. Yesterday appointed Bonnie F. Johns, a county mental health official, to fill the County School Board seat left vacant by the death of Jessie J. Warr Jr. last month.

The executive's spokesman John Lally, said Kelly "believes Bonnie Johns is an excellent replacement for Jessie Warr."

Lally said he expected the County Council to approve John's nomination, in part because of her ability to work with the community and to handle tough issues.

Johns was a cochairman for the county's "Blue Ribbon" Democratic platform committee in the last countywide election in 1974. Thelma Boyd, one of the two other candidates considered for the appointment, said she was disappointed by John's selection and charged that politics and not qualifications influenced Kelly's choice.

Boyd is a retired Prince George's County School guidance counselor. The other contender for the position was Richard S. Brown, a close friend of Warr and a principal of Lyndon Hill Elementary School.

Boyd charged that she was not selected for the unpaid position because of a county fight over property near a proposed Metro station that she said she and other nearby residents won. She said the county delegation, including Maryland Democratic State Sen. Tommie Broadwater, specifically asked her questions about that issue when the school board appointment was considered.

Broadwater, whom Kelly said he consulted before making his choice, told a reporter he felt Boyd was less capable of withstanding pressure on the school board than Johns.

"I'm afraid they see me as an activist," said Boyd, who has served on both the county and state boards of the League of Women Voters. "It's because of this," she added, "that they don't believe I can deal with explosive issues."

Lally said politics had very little to do with the selection of Johns.

The selection of Johns apparently will maintain the moderate balance that the school board has had since last November's election.

Johns, who is black, will be representing the predominantly black district 6, which includes the Fairmont Heights area of the county. She will finish Warr's term which expires in 1980.

The 47-year-old school board nominee plans to keep her job at the Maryland State Health Department, where she is director of mental health consultation and education.

Johns told a reporter it is ironic that she was appointed to the school board post to replace Warr, because she originally came from his hometown, Memphis, and attended his high school.

Johns, like Warr, was instrumental during the desegregation of county schools in 1973. She worked in a volunteer program called Education Action Team that was funded through federal, state and county funds.

The program, she said, was designed to involve both students and parents in the newly integrated system to avoid racial tension.

Johns received a bachelor's degree in 1949 from Southern University in secondary education and a master's in 1952 from the University of Iowa in communication skills.