A Prince William County jury convicted a Gainesville man yesterday of first-degree murder in the death of his wife, whose body was found in an abandoned house in July 1986, nine months after she disappeared.

The 12-member jury deliberated about 90 minutes in convicting Lewis Edward Johnson, 27, and sentencing him to life in prison for killing his 23-year-old estranged wife, Evanda.

Evanda Johnson had planned to testify against her husband in a criminal case in Prince George's County in which Lewis Johnson was charged with attempted murder for allegedly shooting his wife in the neck. After her disappearance in October 1985, he pleaded guilty to a lesser charge.

Circuit Judge Herman A. Whisenant Jr., who presided over the two-day murder trial in Prince William, is expected to sentence Johnson Oct. 16. Under Virginia law, Whisenant may reduce the jury's sentence but may not increase it.

In testimony yesterday, Tonya (Cassie) Haskins, a 17-year-old self-described runaway and acquaintance of Lewis Johnson's, said he came to her on Oct. 19, 1985, and asked: " 'What would you do if I told you I killed somebody? . . . Would you go to the police?' I said, 'No.' "

Haskins said she did not take the conversation seriously until Lewis Johnson took her to his Manassas apartment. "I sat on the bed and he went to open the closet," Haskins testified. "I saw the legs. I told him to close the closet door. I started crying . . . He started laughing."

Haskins said Lewis Johnson told her she had to help him dispose of the body.

Haskins, who worked out an agreement with the commonwealth attorney's office and will not be prosecuted for her actions, said she sat on Johnson's couch while he wrapped the body in a green blanket. The body was then placed in the back seat of Johnson's car and taken to the abandoned house on Carver Road in nearby Gainesville, Haskins said. Evanda Johnson's body, which was found in a closet at the house in July 1986, was so badly decomposed that a cause of death could not be determined.

Commonwealth's Attorney Paul B. Ebert alleged that Lewis Johnson strangled his wife to avoid being punished for the shooting incident in Maryland.

In presenting the case for a charge of first-degree murder, Ebert argued that Lewis Johnson's actions were malicious, deliberate and premeditated. He said a medical examiner had testified that it takes at least two minutes to strangle someone. "What a horrible way to go," Ebert told the jury in his closing argument. "Look at the clock," he said, pausing. Sixty seconds passed in silence. "That's a minute," Ebert said. "Twice that time he was pushing against her throat."

Defense attorneys Lon Farris and E. Allen Newcomb argued that Lewis Johnson, who did not take the witness stand, should be convicted of the lesser charge of manslaughter because his actions did not involve malice and were not premeditated.