The Washington Suburban Sanitary Commission appointed a new general manager yesterday with the hope that he can spruce up the poor image of the bicounty water and sewer agency.

Robert S. McGarry, 46, a former top officials of the U.S. Army Corps of Engineers and the Army's Computer Systems Command, signed a three-year contract with the agency yesterday and will begin work April 18.

His salary will start at $48,000 annually and increase to $50,000 during the term of the contract.

McGarry replaces retiring general manager Robert J. McLeod, a 39-year employee whose last years as general manager were filled with controversy.

Commission Vice Chairman Dean H. Hill, referring obliquely to those controversies, remarked at the end of the morning's news conference, "I'm hoping (McGarry's appointment) gives the agency a new image. We've never been as bad as our image (in the local area). As soon as we get 50 miles away from home, we see that. Our reputation is very good nationally.

McGarry, in explaining why he retired from his rank of U.S. Army brigadier general and deputy commanding general Command to take the WSSC post, cited the agency's "reputation as a very professional organization."

McGarry had been at the Army's Computer Systems Command headquarters at Ft. Belvoir, where he was one of three generals directing the service's worldwide information system. He had served there less than a year.

For three years before that he was the district engineer for the Army Corps of Engineers in Baltimore. McGarry managed a civilian staff of 1,700 persons and supervised the corps' engineering and construction activity for a 46,470-square mile district that stretched from southern Virginia to New York state and from the Atlantic to parts of Kentucky and Indiana. He was also responsible for the port of Baltimore and the District of Columbia water supply.

Observers form both environmentalist and development camps gave McGarry high marks for his ability and integrity.

McLeod announced his intention of retiring in February, 1976, after a Montgomery County grand jury indicted five WSSC inspectors and four construction firm officials on multiple counts of bribery and attempted bribery. He steadfastly maintained that his announcement had no connection with the grand jury action.

The indictments later were dismissed on grounds that the Montgomery County grand jury that issued them had no right to do so, since the alleged criminal violation occurred in Prince George's County. In addition, the court ruled that the state bribery status were improperly applied in the case.

The agency was again the focus of controversy in December when, at a closed session, the commissioners hired McLeod as a $75-an-hour consultant, an apparent violation of its own rules. The consultant contract is in addition to McLeod $38,000 a year WSSC pension. McLeod was paid $54,000 as a general manager.

In an apparently unrelated action, the commission announced that its general counsel since 1974, Paul T. Sisson, had been replaced. Sisson will remain on the legal staff as a senior attorney, commission chairman Johanna S. Norris said. He will be replaced by Paul J. Hefferon, a senior staff attorney.