Mary Conroy, widow of state senator Edward T. Conroy, has been selected by the Prince George's Democratic State Central Committee to fill the Maryland House seat vacated by Gerard F. Devlin. Devlin has been appointed to a District Court judgeship. Conroy will represent District 23, which includes Greenbelt and Bowie.

The committee chose Conroy from among four candidates in an 18-to-1 vote. Some Democrats complained earlier that party leaders had decided long ago to give the post to Conroy, precluding any other nominations.

But, said Del. Gary Alexander, committee chairman, Conroy "received unified support from the three central committee members in her district . . . . She also received widespread community support. Our position is that she'll do a good job.".

Conroy, of Bowie, filled her late husband's seat for six months in 1982 and since last year has been a member of the Prince George's County Board of License Commissioners, which oversees liquor licensing. Union Councils Back Baker

Two statewide councils of the American Federation of State, County and Municipal Employees have endorsed Russell (Tim) Baker for Maryland attorney general. Council 92, which is composed of local units representing state hospital, highway, corrections and other workers, and Council 67, representing local and municipal employes, announced their support at a news conference last week.

The councils represent about 30,000 employes, union officials said.

Ernest Crofoot, executive director of Council 67, said the union leadership believed Baker "understands collective bargaining" and would remain independent in the office.

Baker is one of four Democrats running for the office. Nichols No Longer a Candidate

J. Hugh Nichols, the Howard County executive who earlier dropped out of the governor's race, said last week that he is no longer a candidate for the U.S. Senate or any other office.

Nichols, 56, who switched from Democrat to Republican about a year ago, said his major problem is finding another job after he returns from a trip to Europe.

Nichols was one of the first candidates to enter the race for the Democratic nomination for governor but quit early when polls showed he was running far behind three other candidates.

A year ago he became a Republican in hopes of gaining the party's nomination for governor. His revived effort lasted until January, when he again dropped out of the race, citing lack of party support.

Nichols then contemplated trying to gain the Republican nomination for the U.S. Senate seat being vacated by Republican Sen. Charles McC. Mathias Jr.. The crowded race among the Democrats includes Gov. Harry Hughes, Reps. Barbara A. Mikulski and Michael D. Barnes and Baltimore County Executive Donald Hutchinson. Republicans seeking the office are Linda Chavez, Richard Sullivan and George Haley.

Nichols, who said he has received inquiries from federal, state and local offices and private industry, said he will make his decision in July. Candidate Debates Planned

The League of Women Voters announced last week that it will stage a series of debates on WBAL radio, beginning July 10, for candidates in the gubernatorial primary and other contests.

The first scheduled debate is for the Democratic candidates for governor. Attorney General Stephen H. Sachs has been trying to lure Mayor William Donald Schaefer, the front-runner, into a debate. But Schaefer has not indicated any willingness to participate in such an encounter.

The debates will last two hours and will be moderated by talk show host Joe Lombardo, league officials said.

Debates will be held every Thursday evening through Aug. 28. In addition to the gubernatorial debate, there will be debates between candidates for attorney general, for U.S. senator and for U.S. representatives in the 2nd, 3rd and 7th districts.