When Marvin Mandel was governor of Maryland and state legislators asked for a list of patronage job vacancies, they were curtly refused. "We wanted those jobs to play with," recalled a Mandel aide. "If you play your cards up, you lose your leverage, your mystique."

This year, the mystique is gone. Several weeks before Gov.-elect Harry R. Hughes submits his annual "greenbag" of appointments for State Senate confirmation, the list of vacancies is being handed out to anyone who asks: legislators, women's groups and minority representatives.

There has been no mistake, no unauthourized leak of the treasured information. In fact, much to the amazement of State House veterans, the list was ordered copied and distributed by the woman chosen by Hughes to handle appointments during his transition, Louise Keelty.

Keelty has been raising eyebrows since taking the job after the November election. A former nun turned lawyer and political activist, she has shown little interest in the traditional use of patronage to reward campaign supporters and cement ties to new political allies.

Instead, she says, she intends to remove politics from the appointment process, to hire competent people previously excluded from government work because of their political affiliation and "to give everybody a chance, not just that closed (political) circle."

To widen the circle, she sent hundreds of letters to almost every elected official in Maryland, Democrat and Republican, seeking their recommendations for cabinet posts, boards and commissions. A similar letter was sent to dozens of special interest and citizen groups.

When powerful legislators used to special treatment from the governor's office call Keelty to recommend a friend for a job, she politely thanks them and then places their suggestions in the same file as names of applicants without politicl pull.

Keelty's openness has been applauded by many politicans as refreshing and in keeping with Hughes' campaign promises. But some seasoned insiders call her approach naive and likely to cause more trouble than good for the new governor in the long run.

"The idea is to make more friends than enemies," explained a State House official. "Opening up the process sounds good for while. But if every senator and every delegate sends in 10 names for a job and only one gets it, the rest are going to be mad as hell. They're going to be saying, "What's the matter with my guys?'"

The spoils of victory in Maryland have traditionally gone to friends and supporters of the victor and were used to help him consolidate power once in office. The state Constitution assures the governor of Maryland more power than the chief executives of most states by giving him the right to fill 3,000 jobs reaching as far as local school, liquor and plumbing boards.

In the Mandel years, the patronage tradition was refined to an art form. Mandel sprinkled his friends throughout state courts and commissions and gave others veto power over certain sensitive appointments. He maintained strict control over the General Assembly by allowing cooperative lawmakers to select their friends for county boards.

Mandel's patronage officers were the most active and effective administration lobbyists during legislative sessions. They scurried from legislator to legislator on crucial votes, lining up support here, promising jobs there.

Hughes, a legislative veteran of 16 years who served as Senate majority leader in the 1960's, is no stranger to General Assembly politics. But he has said that he opposes use of patronage as a bargaining chip for favorable votes on bills.

The best sign that Hughes will keep his promise, aides say, is his selection of Keelty as interim appointments officer. She is known for strict ethical standards, a woman committed to bringing more women and minorities into state government even if they cannot help strenghthen Hughes' hand in the legislature or gain favor with political bosses. She says she wouldn't lobby for administration bills if she is appointed permanently.

"If deals are made," she said in an interview last week, "they won't be made by me."

Keelty, 40, brings an unusual background to her job. While the past two patronage chiefs were learning politics from the likes of Lyndon Johnson and Marvin Mandel, she spent her early adult years as a nun in the Sisters of Bon Secours nursing order. She left the convent after six years unable to commit herself fully to the strict religious demands of the order.

She continued to work as a private nurse, but turned to law in the late 1960s and recieved a law degree in 1972. She has served as a Legal Aid lawyer and an assistant state attorney general. Keelty became president of a liberal Baltimore Democratic club in 1974 and served as Democratic committeewoman from Baltimore. She worked for Hughes in the general election campaign.

Politicans who know Keelty say she would make an excellent appointments secretary, especially if Hughes is serious about ending the spoils system. But there is mixed opinion about the new governor's willingness to follow through on his promise if faced with a close vote on a bill he considers crucial.

"Once he's taken office and wants a bill to get through, he'll change his mind," said Sen. Thomas V. (Mike) Miller (D-Prince George's). "Harry Hughes is a good old boy dressed up like a white knight. He's not going to change the rules of the game." CAPTION: Picture, Louise Keelty: "Give everybody a chance . . . " By Tom Allen-The Washington Po