Prince George's County Councilman Francis B. Francois firmed yesterday that he will resign his $24,000 council post on Sept. 5, to join a transportation lobbying group in Washington.

Francois said his decision to resign after 18 years in county politics was motivated by a desire to get more involved in national and state issues and by personal financial considerations. As executive director of the American Association of State Highway and Transportation Officials he reportedly will earn $55,000 a year. h

"I always felt my name was written in pencil on that [council] nameplate and it's time to erase that and write someone else in," the 46-year-old Bowie Democrat said.

Francois's decision to quit politics took many county Democrats by surprise. It also touched off what is certain to be a summer-long flurry of activity among the party ranks to find a replacement to finish out the two years remaining in Francois' term.

His decision also threw open the 1982 Democratic primary for county executive. Until yesterday dozens of interested Democrats had curtailed their ambitions because of wide support among the party for a Francois candidacy.

According to the county charter, Francois's successor must be chosen by the county council within 30 days of his resignation. The council makes the selection from a list of three names submitted to them by the Democratic Central Committee, since Francois is a Democrat.

Although most council members were hesitant to discuss publicly a possible replacement for Francois, a handful of names began to surface by yesterday morning.

Francois yesterday endorsed one of his aides, Judy Colbert, as his successor. Also mentioned as possible candidates were A. James Golato, a member of the county school board; former police chief John Rhoads; and John Lally, a lawyer who was an aide to former county executive Winfield M. Kelly and ran unsuccessfully for council in 1978. Lally has said publicly that he is not interested in the position.