Newly elected Montgomery County Executive Charles W. Gilchrist has appointed Paul A. McGuckian, a Rockville lawyer, as county attorney, effective Jan. 1.

McGuckian, a Democrat who was defeated earlier this month in his bid for the House of Delegates from the 17th District, will replace Richard S. McKernon, who was appointed county attorney by outgoing Republican executive James P. Gleason in 1971.

McKernon will join a Rockville law firm that is organizing Jan. 1 under the name of Mannes, Myers, Nadonley, Townsend, McKernon and O'Brien.

McGuckian, 40, has been counsel to the county's state legislative delegation since 1974, during the same period that Gilchrist, also a Democrat from Rockville, was a state senator. McGuckian was an assistant county attorney from 1966 to 1971 and later was counsel to the county Housing Opportunities Commission.

He is a partner in the Rockville firm of Trimm, Donohue, McDanald, Willis and McGuckian.

The county attorney, the government's civil lawyer, supervises a staff of 20 attorneys.

Gilchrist also has decided not to reappoint Ronald Lloyd, who has been the county personnel director for six years. Gilchrist has not named a successor, but Lloyd has resigned effective tommorrow. He is reportedly assuming a similar position in another jurisdiction outside the area.

Earlier, Gilchrist appoint Thomas Stone, 31, as a special assistant Stone, one of Gilchrist's cheif campaign aides, is helping supervise Gilchrist's transition office and will be one of the new executive's "troubleshooters." He was an assistant chief of park planning for the Montgomery County Planning Board.