Marvin Mandel can still be found most mornings in the corner booth at Chick and Ruth's Delicatessen on Main Street in Annapolis. His political advice is still a sought-after commodity. And, to some extent, he is still in the public eye.

But in the three years since the former Maryland governor stepped off an airplane at Baltimore-Washington International Airport after completing a 19-month prison term for mail fraud and racketeering, much has changed for the man who ruled the state as few have.

Mandel seems to have put his former life behind him comfortably, dwelling neither on his lost power nor on the notoriety that engulfed him. "In a sense it has been an easier transition than I ever thought it would be," he said.

He keeps busy with two jobs, one with an Anne Arundel County development firm where he analyzes projects, the other with a five-day-a-week radio show on an Annapolis station in which Mandel interviews people as diverse as the publisher of the Baltimore Sun and experts from the Smithsonian.

And he is doing something he had little time to do during his 28-year public career: spending considerable time with his family. "That's the most important aspect" of his changed circumstances, he said.

All in all, Mandel said, he is "very comfortable" being a private citizen.

But the former governor says he maintains an active interest in the public and political affairs of Maryland. "I guess I'll never lose that," he said, although he said he envisions no imminent return to political life.

Still, Mandel continues to have a stubborn hold on the public imagination. Every few months rumors reemerge that Mandel is dipping his hand into politics or the legislature. Though Mandel denies such stories, they persist, as if few people can believe that someone once so powerful could refrain from wielding influence behind the scenes.

For example, during the recent presidential election, rumors were rife that Mandel was operating quietly on behalf of President Reagan's reelection. After all, the talk went, it was Reagan who commuted Mandel's prison sentence, and it would be only logical for Mandel to return the favor.

Mandel himself said it was not so. But he did spend part of election night in a back room at the state GOP election night headquarters in Annapolis, according to people who were there.