Robert S. McGarry decided yesterday against quitting his job as top executive of the water and sewer agency that serves Prince George's and Montgomery counties, his associates reported. His change of mind came after he got a vote of confidence from the Prince George's County Council.

McGarry said last week he would not seek reappointment to the $50,000-a-year job as general manager of the Washington Suburban Sanitary Commission because of political feuding between the two counties.

He said at the time that the persistent feuding had made his job increasingly difficult for the last nine months and prevented the resolution of many major issues pending before the WSSC.

Another apparent factor in his decision were reports that Prince George's officials were unhappy with his performance and felt he had favored Montgomery too often.

The Prince George's Council held a lengthy conversation with McGarry yesterday, then issued a letter saying McGarry has "carried out [his] responsibilities in both a professional and equitable manner."

The council's support was crucial for McGarry because of a split that had developed among the six WSSC commissioners over his contract. The commissioners are responsible for hiring the agency's general manager.

The three Montgomery commissioners and their county government had favored renegotiating McGarry's contract. As happened on many other issues this year, the Prince George's commissioners and county government disagreed, pressing for more discussion of the general manager's influence and role.

Council members said they hope their support will result in at least one Prince George's vote on the commission in favor of giving McGarry a new contract. McGarry needs four of the six commission votes to get a new contract.

The three Prince George's commissioners, who also met with the council yesterday, said they have not made up their minds on McGarry's contract.

After yesterday's session with McGarry one council official said, "there was a general feeling that with all the other problems and impasses between the two counties, the selection of a new general manager would be disastrous."

In the last nine months, the commission has been split along county lines on almost every major issue, including rate increases, the agency's budget sewer capacity and sludge disposal.