Alan M.Wilner, a chief aide to Gov. Marvin Mandel for nearly four years, has been appointed to a seat on the Maryland Court of Special Appeals, the second highest court in the state.

Acting Gov. Blair Lee, who made the appointment, said Mandel asked him to select Wilner and that he agreed with that suggestion. "Both Gov. Mandel and I are pleased to share credit for the selection of Mr. Wilner," said Lee.

Wilner, 40, joins a growing list of Mandel's friends and political associates who have moved into judicial positions during his tenture as governor. The list includes Court of Appeals Judge John C. Eldridge, who was Wilner's predecessor as legislative aide; Court of Special Appeals Chief Judge Richard P. Gilbert, once a partner in Mandel's Baltimore law firm; and Judges Rita C. Davidson and David T. Mason, both of whom served as cabinet members in the Mandel administration.

Lee assumed the constitutional duties of governor on the eve of the ailing Mandel's political corruption retrial. "It's an appointment I would have made anyway," he said when asked about Mandel's role in the decision.

Later, when it was noted that Wilner has no judicial experience, Lee argued: "Wilner is a scholar of the law. He thinks clearly and writes beautifully. He'll be a damn sight better than a run-of-the-mill judge."

This praise of Wilner was shared by Senate President Steny H. Hoyer (D-Prince George's) who said he had no complaints the appointment despite the fact that he would have preferred someone coming out of his home base in Prince George's County.

"I don't think it's an example of cronyism," said Hoyer. "Alan Wilner is one of the most capable people I've ever met and he'll do an outstanding job. One or two people have called me to complain about it, but I've said to them: 'Look, we're talking about a competent and qualified individual."

Four Prince George's officials were interested in the judgeship: State's Attorney Arthur A. Marshall Jr., attorney Karl Feissner, Circuit Court Judge James Taylor and District Court Judge James M. Rea.

Wilner became Mandel's chief legislative aide in 1973 when Eldridge was named to the hight court. Before that, he had served in the state attorney general's office and in a lesser capacity in Mandel's office. He is a graduate of Johns Hopkins University and holds a law degree from the University of Maryland.

Wilner's appointment will take effect Aug. 1. He will serve until the 1978 general election, when he will be required to run for a 10-year term to keep the job. His seat on the Court of Special Appeals - the 13th on the court - was created by the 1977 General Assembly.