Terrence G. Johnson, the Prince George's County youth convicted of slaying a county police officer in 1978, lost the final appeal of his conviction yesterday when the Supreme Court refused to review his case.

The court's action, which was made without comment, leaves unaltered Johnson's 1979 conviction and 25-year sentence for manslaughter and illegal use of a handgun in the death of county police officer Albert M. Clagett IV.

The court's decision does not affect another legal action that Johnson's attorneys initiated in county Circuit Court last month. The lawyers asked, in a post conviction proceeding, that Johnson be given a new trial because of what they say was an error by the judge presiding over his case. A ruling on that request is expected Jan. 21.

Johnson's attorney Allen Lenchek said yesterday that while he was disappointed that the Supreme Court did not agree to consider the appeal, he was not surprised. "Let's say that there's always a high probability that the court will deny [a request to review a case] because of the large number of [requests] that they receive," he said.

Johnson was charged in 1978 with murdering two county police officers. After an emotional trial the then 16-year-old was acquitted of the murder charges but found guilty of two lesser offenses in the death of police officer Clagett.

The jury found Johnson innocent by reasons of insanity in the death of the other officer, James Swart. Johnson has been serving his 25-year term at Patuxent Institute in Jessup, Md.

Immediately after Johnson was convicted, his attorneys filed an appeal with the state Court of Special Appeals. The lawyers maintained that Johnson, who was 15 when the shooting incidents occurred, should have been tried and sentenced as a juvenile. Because of the seriousness of the charges against Johnson, he was tried and sentenced as an adult.

The state court, after considering the appeal, affirmed Johnson's conviction in February of 1979. His attorneys appealed that ruling to the state's highest court, the Court of Appeals, which in May 1980 refused to consider the case, thus letting Johnson's conviction stand.

It was at this point that Johnson's attorneys took his case to the Supreme Court. They asked the court to consider constitutional issues including whether Johnson was denied due process when he was tried and sentenced as an adult instead of as a juvenile, and whether the circuit court, which tried Johnson, also violated his right by excluding evidence regarding the reputation of the county's police department in dealing with the black community and the reputation of officer Clagett.

Johnson is black and the county's 800-officer police force, which is almost all white, has been accused in the past by members of the black community of using excessive force.

Lawyer Lenchek said that the final hope for Johnson now lies with the post-conviction hearing and a request that he be given a new trial because of a previously undisclosed conversation between the presiding judge at his trial and a juror in the highly publicized case.