Prince George's County Executive Lawrence J. Hogan, saying there was a "vital need for minority representation in the upper ranks of the police department," yesterday appointed Thomas M. Davis, a black civilian police employe, to be a deputy chief in the department.

The appointment becomes effective July 1 when the number of deputy chiefs is increased from one to four. Davis and the three other deputies will rank below Chief John E. McHale.

Davis, 50, has worked for the police department for six years, teaching at the County Police Academy and designing classes for recruits. As a deputy chief, he will be in charge of personnel. He said yesterday he plans to expand recruiting beyond Prince George's and Montgomery counties into Pennsylvania and the Carolinas.

Davis's appointment follows years of criticism of the department led by the local NAACP. In 1976, the U.S. Justice Department filed suit charging that the Prince George's Police Department discriminated against blacks in hiring. The county bitterly contested the suit, refusing for nearly two years to negotiate with Justice attorneys.

The suit was finally dropped because county officials showed they were making serious efforts to recruit blacks.

In 1975, the 825-member police force was less than 5 percent black; today, an 850-member force is 12 percent black, but none has rank above sergeant. The county's population is 36 percent black.

In announcing Davis' appointment, County Executive Hogan said, "Even though I am convinced we do not have a problem of racism or brutality in the department, this is the culmination of an effort in trying to address the problem, real or imagined."

The appointment of a black to a high ranking police position was made possible last year when voters approved a referendum proposed by the County Council authorizing three deputy chiefs who could be outside the county merit system and therefore would not have to rise through the ranks.

As it happened, Davis will be the only deputy chief to receive a political appointment, which carries with it the rank of lieutenant colonel; the other three are veterans of the force and already hold the rank of lieutenant colonel as McHale's chief assistants. The current lone deputy chief, Lt. Col. Rice Turner, will join Lt. Cols. Joseph Vasco and Vincent duCullier plus Davis at the deputy level July 1.

McHale said that although Davis will be assigned to personnel, the duties of the other three deputies are still being determined DuCullier now is in charge of personnel, records, data processing and purchasing. Vasco is in charge of detectives and uniformed police, and will lose some of his authority to Turner, according to Turner.

Davis, whose new job will pay $40,000 a year, is a retired Army lieutenant colonel and is from North Carolina.

Laney Hester, president of the county's Fraternal Order of Police chapter, said yesterday the union is considering filing suit against the department because "we don't believe the referendum [that authorized additional deputy chiefs] was intended to have some personnel of the same rank exempted from the merit system and some who are not."