John Kevin Johnson, a 26-year-old laborer, was convicted early today of first-degree murder, rape, kidnaping and five other offenses in the slaying of a 13-year-old Southeast Washington girl last February. Hearings were scheduled for next month to determine whether he should be sentenced to die in Maryland's gas chamber.

A Prince George's County jury of seven women and five men deliberated more than seven hours before delivering its verdict about 12:15 a.m. Among the crimes it found Johnson guilty of were felony murder, robbery, two sex offenses and use of a handgun while committing a felony.

Johnson, jailed without bond, remained calm as the verdict was read, climaxing a three-week trial in the killing of Arlene Flowers, a standout basketball player at Hart Junior High who lived in Washington's Valley Green public housing project with her mother and six brothers and sisters.

Shortly after midnight last Feb. 20, the girl's brother, Morell, returned home from work and asked her to go to a nearby convenience store to get him a soda and some ice cream, according to testimony at the trial.

She disappeared, and her body was found March 2 in the shallow waters of Mattawoman Creek on the Prince George's-Charles County border. She had been killed by a shotgun blast to the back at short range that pierced her heart.

A county prosecutor told the jurors that Johnson might have escaped arrest in the slaying of Arlene Flowers of Valley Avenue SE if he had not told five persons about the crime in vivid detail. The five were the prosecution's key witnesses during the three-week trial.

Defense attorneys countered that there was no direct evidence such as fibers, fingerprints or chemical traces linking Johnson with Flowers. "There is not one single match to be found in this case," said attorney Darlene Perry. "The sole evidence comes from the mouths of other people."

Flowers' body was found March 2 in the shallow waters of Mattawoman Creek on the Prince George's-Charles County border. She had been killed by a shotgun blast to the back at short range, which pierced her heart.

In his final presentation to the jury, Assistant State's Attorney David Simpson displayed a sawed-off shotgun bearing Johnson's name that police contended was the murder weapon, as he argued that Flowers had been "forced to disrobe" and then there "began the most horrible series of events that can happen to a human."

Simpson said details were learned from the five key prosecution witnesses, and that Johnson had told three of them about the slaying within four days after Flowers' death.

One of the witnesses, Monty Montegute, 25, a disc jockey who lived in a Southwest Washington apartment where Johnson sometimes stayed, testified he repeated the story to D.C. police, and Johnson was arrested along with another man on March 13. Charges against the second suspect were dismissed the following month since authorities had only Johnson's word linking him to the slaying, according to police.

Charles Price, who works in Johnson's uncle's auto parts yard, testified Johnson told him that he and the other man who was initially charged took turns raping the girl in Johnson's trailer behind 17806 Livingston Rd., in Accokeek, Md., for four to six hours. He said Johnson told him Flowers continually begged for mercy and called for her mother up to the moment Johnson let her go on Sharpesville Road near the creek, telling her to walk home. Price testified Johnson told him he then shot the girl in the back.

Richard Carlyle Harrington, a convicted murderer and armed robber, testified Johnson told him of killing Flowers while both men were in the county jail. He also said he went to police after Johnson tried to arrange for the murder of three potential witnesses at the trial on Flowers' murder.

A police detective, Roland Sweitzer, testified that when he went to the jail posing as a hit man, Johnson signed over title to a 1967 Oldsmobile and promised an additional $2,000 for the murders of Montegute and two other persons.

Johnson has been indicted also on murder solicitation charges for which he will be tried separately.

Defense attorney Perry argued that Montegute lied to police to protect himself from suspicion and insisted that other witnesses against Johnson could have heard the story of Flowers' rape and murder through the news media.

Johnson, who sat calmly through his trial and frequently participated in discussions with his attorneys, did not testify in his own defense.

Last year he was convicted in D.C. Superior Court of assault with intent to kill in an incident in which several shots were fired at a Southeast Washington woman. According to court records, he was given 24 months' probation.