The man who authorities say provided the key to solving the murder of Catherine Fuller was sentenced yesterday to serve at least eight years in prison after an unusual and emotional plea for leniency from the prosecutor.

"It is most important to understand . . . that there would have been no case without" the cooperation of Harry J. Bennett, prosecutor Jerry S. Goren said as he urged the judge to take into consideration Bennett's role in the convictions of 10 other defendants in the brutal beating death of the mother of six during a robbery attempt.

"I'm saying all of this not because I think Mr. Bennett should be rewarded . . . but that it not be forgotten that he attempted to do the right thing," Goren said.

Bennett pleaded guilty to manslaughter and robbery charges and agreed in April 1985 to testify against the other defendants. At that time, prosecutors had 13 persons in jail charged with Fuller's slaying, but had almost no legally permissible trial evidence or witnesses. Bennett's plea represented the government's first major break in the celebrated case and Goren, who resigned recently as an assistant U.S. attorney, told D.C. Superior Court Judge Stephen F. Eilperin yesterday that he had returned to court to fulfill the promise he had made to Bennett that April day.

Judge Eilperin told Bennett yesterday that he recognized the help he had provided the government, but that his involvement in the crime could not be overlooked. The judge also told Bennett that fairness dictated he consider the sentences of 35 years to life imposed on most of the other defendants, and added that Bennett had already been offered a "bargain" for his cooperation when the government let him plead guilty to reduced charges.

Eilperin said that as a result of the government's request for leniency he would take "20 percent" off the maximum 10 1/3 to 31 years in prison Bennett faced for the Fuller charges as well as a separate drug possession case. Eilperin sentenced Bennett to a minimum of eight years in prison and no more than 30.

"When Mrs. Fuller was crying out for help that didn't deter Mr. Bennett," the judge said.

Unlike the other sentencings, yesterday's session brought a somber mood to the courtroom as many of the detectives assigned to the case crowded the front row to provide support for Bennett.

Moments before the judge took the bench, homicide Detective Patrick McGinnis quietly tapped Bennett on the shoulder and shook his hand, wishing him well. Barbara Wade, Fuller's sister, leaned over and thanked Bennett for helping solve the murder.

Bennett, who had been free on bond pending his sentencing, entered the lower-level courtroom clasping a plastic bag of toiletries and tennis shoes. He sobbed and covered his face as he told the judge how sorry he was for his involvement in the Oct. 1, 1984, Fuller slaying. His girlfriend Trina Ward abruptly left the courtroom.

Prosecutors contend that Fuller was beaten to death in an abandoned Northeast garage during a robbery attempt as dozens of young people watched. Nine young men and one woman were tried last year for the slaying -- the largest number ever tried for a single homicide in the District -- and eight were convicted. Two other persons eventually pleaded guilty.

"I'm sorry for what happened to Mrs. Fuller . . . , " the 20-year-old Bennett said, standing next to his lawyer Earl Zack. "I tried to do the best thing that I could . . . .

"I never had a family. I never had nobody but myself," he said, referring to the incarceration of his mother and father during his childhood.

"Now I do have a family . . . . I've got a job . . . . I should be punished but I also think . . . . " He began crying and his voice trailed off.

The final defendant in the Fuller case, Calvin Alston, is scheduled to be sentenced today.