Marvin Mandel, whose long political career was interrupted 17 months ago by a conviction on corruption charges, reclaimed the full powers of the Maryland governor's office this afternoon for the final 45 1/2 hours of his term.

"It's a very sentimental and touching moment for me," said Mandel, whose political resurrection began four days ago when an appeals court in Richmond reversed his conviction and spared him a four-year prison sentence. "I came to the office of governor with a great deal of pride and I want to leave as I came in."

Mandel was sitting in a front row seat in the House of Delegates chambers, listening to Louis Goldstein deliver his inaugural speech for a sixth term as state comptroller, when the clock struck half past the hour of two and he became, once again, Maryland's governor in function as well as name.

His first act in this renewed role was to slip into a back room of House Speaker Benjamin L. Cardin's office and confer privately with Blair Lee III, the acting governor during Mandel's exile, and Harry R. Hughes, the man who will take over as governor at noon Wednesday.

The three men, all of whom had a share of the gubernatorial title in one form or another over the last week, stayed in the room together for 20 minutes. They emerged with smiles on their faces and little to say.

"We talked about the weather," said Mandel, after his first face-to-face encounter with Hughes since the governor-elect won office by promising a change from the corruption-tainted Mandel administration. "I'm serious about that -- that's about all we talked about."

Lee said the meeting involved "the simplest kind of chit-chat you've ever heard in your life." At one point, said Lee, Mandel offered Hughes "some fatherly advice, but it was totally unmemorable."

Hughes said he went into the room to discuss legislation with Cardin. His encounter with Mandel, Hughes said, "was certainly not unfriendly."

And so the short second reign of Gov. Mandel -- something that had been anticipated here as though it rivaled the return of Napoleon from Elba -- began with a closed-door meeting dominated by trivial conversation.

After the session, Lee slipped away to get a haircut from barber izzy Wolfe on Main Street, Hughes stepped outside to take a look at the construction work going on for his inauguration, and Mandel took the elevator up to the second floor where he met again behind closed doors, with former staff assistants.

Mandel spent the day precisely as he said he would at the morning press conference at which he announced he would reassume power. "I'm going to enjoy the sights of Annapolis," he said then. "I'm going to take telephone calls, meet with friends and associates, greet well-wishers."

Although his title as governor was automatically restored Thursday night when the federal appeals court overturned his August 1977 conviction, Mandel was not legally authorized to function as chief executive without first rescinding a letter he signed 19 months ago transferring the full powers of governor to Lee.

Regaining control of state government has little practical significance for the 58-year-old Mandel just two days before Hughes enters office. Lee and Hughes have completed work on all the available patronage appoint-the 1980 budget, and Lee has used up ments to state boards and commissions.

Mandel, who pulled the strings of government like a master puppeteer in his eight years in office, said he would allow the state to run itself for the next 48 hours. Whatever powers remain -- pardons, staff hirings and calling special sessions of the legislature -- will not be exercised in his brief remaining tenure, he said.

"I'm not trying to prove anything," he said, as if to quell speculation that he would use his final hours in office to get back at political enemies and reward old friends. "Let me make one thing clear so we don't get into any complications at all. I'm not going to hire anybody. I'm not putting anybody on the payroll. I'm not adding any staff. I'm not doing any of those things and, honest to God, you'll just be wasting your time if you [reporters] run around to [former administrative officers] Hans [F. Mayer] to see if he has the [payroll] book out or check with the payroll clerk."

Then, he left himself an opening: "I was never a very inactive governor and I won't be if a problem arises."

The details of Mandelhs return were worked out during an hour-long meeting at Lee's Silver Spring home Sunday, although the precise timing for the transfer of power was left until this morning.

The two men, politicians of greatly varying backgrounds and styles who have guided Maryland government for the past decade, met in the governor's office across an old stenographer's desk that was carted into the vacant room for the historic occasion.

Finally, they agreed on a 2:30 p.m. termination for Lee's service as acting governor, giving him just enough time to complete Goldstein's swearing in before hundreds of spectators in the House chambers. The letter was signed shortly before Mandel walked out to address the press at 10:30 a.m.

Mandel, who has displayed a sharp sense of timing throughout his political career, was prepared for any delays this afternoon as he sat in the chambers, his wife Jeanne at his side, and watched the Goldstein swearing-in.

When the time had reached 2:15 and the ceremony had yet to pass the introductions stage, Mandel calmly took a sheet of paper out of his pocket and waved it at Lee, who would have the power to swear-in Goldstein for only another 15 minutes. "I was prepared," said Mandel later. "The letter in my pocket would have extended Blair's time in office for a few minutes."

Lee was able to conduct the swearing-in with minutes to spare. The deed accomplished, he joked: "I've been keeping a close eye on the clock. Louis, you are sworn in and legitimately sworn in." Then Lee noted that the rabbi who delivered the invocation left a "pregnant pause when he said 'Bless our governor.' I thought he was going to add -- 'whoever he may be'."

Lee, a Montgomery County politician who served as Mandelhs lieutenant governor for seven years, has remained mildly amused by the State House commotion of recent days, acting at times as if he enjoyed falling back into his old role as number two man.

"I'm continuing to roll with the punches," said the 62-year-old Lee, who believes his longtime association with Mandel caused him to lose the Democreatic primary for his own term last summer.

Lee's most perplexing problem since falling from power, he joked, was finding a place to sit in a State House in the middle of three administrations. With Mandel taking his place in the governor's office and Secretary of State Fred Wineland occupying Lee's old lieutenant govenor's suite, Lee said, "I may be wandering around the building looking for a chair."

Just about everything that happened in and around the State House today had more symbolic than real meaning. Mandel visited the governor's mansion at noontime, but only to pose for pictures and greet the staff there, not to move in.

Several of Mandel's former aides -- including patronage secretary Maurice R. Wyatt and lobbyist Frank Harris -- were roaming the halls of the building all day, but they insisted that they had just come back to watch their old boss take over, not to help him do it. They were subdued and stayed in the background today, unlike Friday, when they had charged into the State House shouting: "The Muldoons are back! The Muldoons are back!"

In the afternoon, workmen could be seen building the outdoor podium and hauling folding chairs onto the State House grounds in preparation for Hughes' Wednesday inauguration. Mandel has not been asked to attend the inauguration, but he insisted again today that he did not feel slighted by the lack of an invitation. "No, gentlemen, I don't feel slighted a bit," said Mandel, who maintained that there is a tradition in Maryland that outgoing governors not attend such ceremonies."And the way I feel right now, a little slighting wouldn't hurt."

Hughes, when asked whether he would give Mandel a last-minute invitation, said: "Well, I understand he would not come, even if invited. He doesn't want to distract from my day. I thank him for that."

Mandelhs dramatic four-day revival forced Hughes' staff to reschedule its State House arrival. Several hours before the court reversal last Thursday, Hughes' appointment secretary, Louise Keelty, called her predecessor to warn him she planned to move into his office today.

"I've been looking for her all day," said Lee's patronage chief, Simon McHugh, as he jokingly searched under his desk. "I just can't find her anywhere."

Lt. Gov.-elect Samuel W. Bogley III was puzzling today over plans by Hughes' aides to meet at the governor's mansion on the morning of the inauguration so they could walk together to the new governor's swearing-in.

"When I knock on that door," said Bogley, "I don't know who's going to be there."