The Montgomery County Council elected yesterday as its president Isiah Leggett, the first black council member in Montgomery history, and County Executive Neal Potter began changing the executive branch of government.

Potter appointed Maryland Assistant Attorney General Joyce Stern as the county attorney and former state senator Edward J. Kasemeyer to be the county's chief lobbyist to Annapolis.

The appointments, along with the council's confirmation yesterday of William Hussmann for the $105,000-a-year post of chief administrative officer, represent a dramatic change in the face of county government.

By contrast, when Sidney Kramer took over as county executive four years ago, he largely kept the government of his predecessor, Charles W. Gilchrist.

Stern, a former member of the County Attorney's Office, and Kasemeyer, a Howard County Democrat who was defeated for reelection in November, are expected to be confirmed by the council next week.

Stern would be paid $80,000 annually and Kasemeyer $70,000.

Potter confirmed the resignations of Health Department Director Martin Wasserman, animal control Director S. Mark Dubinsky and Robert Slesser, director of information systems and telecommunications.

Transportation Director Robert S. McGarry resigned when Potter was elected.

Potter said he decided against asking for letters of resignation from all department heads.

He said he has asked some department heads to stay, while the future of others, including Police Chief Donald E. Brooks, will be determined in the coming months.

Potter could not make wholesale changes because most of those in the county's work force are merit civil service employees.

Leggett (D-At Large), a prote'ge' and ally of Potter's, was unanimously elected president.

The largely ceremonial post provides higher visibility for the person who holds it, and Leggett will preside over a council that is composed for the first time of nine members, five elected by district.

Council member Bruce T. Adams (D-At Large), another Potter ally, was named vice president.