Former Maryland governor Marvin Mandel started his post-prison professional life yesterday by taking a job with a construction and development company within minutes of his home in the Annapolis area.

Mandel still must return each weeknight to the halfway house in Baltimore that will be his residence until Dec. 20, but the new job will shorten the number of hours he must spend there each day and allow him to have weekend furloughs.

"I'm delighted to be working," Mandel said outside the offices of Charles J. Cirelli & Sons, where he will be dealing with proposed real estate ventures. "I think it's going to be the kind of thing I'd like to be doing, with something going on constantly: challenges, new things, new opportunities."

Last Thursday, President Reagan commuted Mandel's three-year prison term, cutting nearly five months from the sentence of the onetime chief executive of Maryland who was convicted in 1977 on political corruption charges. Reagan set the Dec. 20 release date and also ordered Mandel's immediate transfer from a Florida prison camp to a Baltimore halfway house.

The former governor was granted a routine furlough for his first two days home, and after spending a quiet weekend with his wife, began sifting through job offers that he said had poured in from friends and strangers. He turned down an offer from Baltimore Mayor William Donald Schaefer to work as a city budget analyst, explaining later that he appreciated the offer but "had no desire to be involved with any public employment."

Instead, he took a job offered by contractor Charles Cirelli, one of his neighbors in Arnold, Md. Cirelli, according to Mandel, had talked about the offer with the former governor's wife Jeanne.

The Cirelli company, a family-owned operation that employs between 75 and 125 people, develops private commercial and industrial projects and bids on public projects in several counties, according to company officer Jack Leone. The company has won contracts in past years from the city of Baltimore, according to Richard Lidinsky, clerk to the city Board of Estimates.

Mandel said he spent yesterday "getting acclimated" at his new office and planned to take some files of "homework" back to the Volunteers of America halfway house, where he must return at 10 p.m. each weeknight.

The 61-year-old Mandel said that after his first few days of freedom, "I feel like a new person."

But despite the calm of his first few days back in Maryland, the former governor still has legal problems to face. The state of Maryland has accused him in a civil lawsuit of removing state-owned property from the governor's mansion when he moved out in 1977. And Mandel, currently suspended from practicing law in Maryland, will likely face disbarment proceedings based on his federal mail fraud and racketeering conviction.

The battle over the property in the governor's mansion flared last year before Mandel began his prison term. Maryland Attorney General Stephen H. Sachs accused the Mandels of taking state-owned furniture, household goods and more than $3,000 worth of liquor from the mansion after Mandel's 1977 conviction. The state filed a civil lawsuit seeking the return of the property and more than $20,000 in damages.

Mandel countered with his own lawsuit against the state. In it, he asserted that most of the furniture and bric-a-brac that state officials say he improperly carted away was actually his own property. Furthermore, Mandel contended, the state had 18 pieces of his personal property, which he wanted back.

Both cases, stalled by Mandel's imprisonment, are likely to move forward now.

As for Mandel's law license, the counsel to the Maryland Attorney Grievance Commission said the agency would file a disbarment petition with the Court of Appeals. "We plan to give the guy a couple of breathing days," said bar counsel Melvin Hirshman, "but we are planning to do our job."