High above Baltimore on Delta Flight 150, Marvin Mandel began his personal countdown to freedom. He craned his neck. His eyes darted back and forth between two little airplane windows, straining to see through the thick clouds. He pointed out landmarks. He checked his watch. He smiled wistfully. He called out the minutes left to landing.

"In just a couple of seconds we'll be on the runway. Here we go," the former Maryland governor exclaimed with the gleeful anticipation of a little boy, as the plane screeched onto the pavement at Baltimore-Washington International Airport yesterday. "Oh boy. Holy mackerel."

Just 24 hours after President Reagan commuted his three-year prison term, Mandel came back to his home state in a six-hour journey that had all the trappings of Napoleon's return from Elba.

When the plane landed, state troopers cleared it of reporters so Mandel and his wife Jeanne could share a private greeting, and then the smiling pair strode off arm in arm. They were greeted by reporters and cameramen and a crush of friends and relatives who devoured Mandel with hugs and kisses.

Then the two-time chief executive of Maryland, who was convicted in 1977 on political corruption charges, stepped to a microphone and stared out at the eager faces and bright television lights.

"You know, it looks like old times," Mandel asserted with a broad grin. And his wife Jeanne looked up lovingly and said, "You're back where you belong."

Mandel's homecoming was set in motion Thursday by the president's executive clemency order. It stated that Mandel should be released from custody Dec. 20 -- nearly five months before his term was scheduled to end -- and that he should be moved immediately from a federal prison camp to a halfway house in Baltimore.

Mandel learned of the commutation at the Eglin Air Force Base prison camp in Florida, where he has been incarcerated for the last 19 months. The news came in a phone call from a friend and resulted in a sleepless night of elation, tears and expectation.

Yesterday morning, Mandel recalled on the plane trip from Eglin to Baltimore, he got up before 5, donned his prison garb of gray sweatshirt and blue work pants for the last time and ate a quick breakfast before he was called to "Control," the inmates' name for the administration building. There he was told, "You're going home as quick as we can get you out of here."

In less than two hours, Mandel was packed, his paperwork was done and with nothing but a shaving kit and a sheaf of personal letters in hand he headed for the prison gates. "They gave me $30, I signed for the money, I shook hands with the superintendent and I walked," Mandel recalled of that sweet moment of liberation.

Reporters greeted him at the Eglin airport and from then on Mandel's trip back to Maryland turned into a free-wheeling press conference in the sky. He said that his Republic Airlines flight looked "more like Air Force One," complete with cameramen jockeying for position and reporters hanging on his every word.

The 61-year-old Mandel, acting alternately like an elder statesman and a little boy, held forth on subjects ranging from prison reform to the 1982 Maryland gubernatorial race. Asked and asked again about his own possible political future, he answered in his uniquely vague style, never foreclosing the possibility that he might some day run for office.

"There's an old Army expression: 'Never cut off your bridges behind you. You may have to retreat that way sometime,' " he later explained of his inscrutable answers.

After finding another clutch of reporters waiting for him during a change of planes in Atlanta and savoring the praise of a stranger who stopped him to say, "Governor, you're the greatest," Mandel made what he called a "little joke."

"The hell with the governorship," he said, "Maybe I'll run for the presidency."

There was also the hint of Mandel, the consummate politician who has never lost an election, as political kingmaker, pushing Baltimore Mayor William Donald Schaefer as a possible gubernatorial candidate. "I think all of you reporters are taking for granted that Schaefer is not going to run," he said, taking a dig at Gov. Harry Hughes, the Democrat who took the State House by running an anti-Mandel campaign. "I think you're wrong. I think he's playing a very clever game. He doesn't have to campaign. Everybody is doing it for him."

Despite the VIP treatment he was accorded at the Atlanta airport by airline officials who whisked him to a special lounge and the attention of the press, it was the little things that seemed to impress Mandel on his first day of real freedom.

"You were made like a robot in prison," he said. "You can't know what it means to go in and buy whatever you want to eat instead of eating what's given to you, to put your hand in your pocket and buy something, to not have to stand in line to get a telephone."

Later, at his press conference, he said, "I've survived it prison . I hope I've come out a little wiser, a little better man. Now, if there are no more questions, I have that chore to do."

With that, he and his entourage swept out of the airport, protected by a phalanx of state troopers and Marvin Mandel headed to the halfway house in Baltimore that will be his residence for the next 16 days.