The U.S. Postal Service Board of Governors, in an unprecedented move, fired Postmaster General Paul N. Carlin yesterday after 12 months in office, amid accusations that he has moved too slowly in cutting the post office's bloated headquarters staff while failing to master key details of his job.

The board, traditionally a rubber-stamp body that has become more assertive since President Reagan's appointees gained control, immediately named former American Airlines chairman Albert Vincent Casey postmaster general.

Carlin's unexpectedly swift firing and the decision to replace him with Casey came during the governors' closed-door monthly meeting yesterday, a Postal Service spokesman said. A formal announcement of the changeover is scheduled today.

Casey, 65, a Harvard-trained businessman, was an executive for two railways and a manager for Times Mirror Co. before becoming chairman of the board and president of American Airlines. As the new postmaster general, he will become the 66th successor to Benjamin Franklin.

Carlin, 54, was a Postal Service insider, a former regional postmaster in Chicago who took over Jan. 1, 1985, by a unanimous vote of the board.

His replacement by Casey was seen as a signal of the board's frustrations with what sources called the post office's internal management malaise and as a commitment to bring in an outside manager to make tough reorganization decisions and create a leaner, more modern and more efficient mail-delivery service.

"Everything from the mid-70s has been to establish a leadership corps that would move into the mid-levels of authority and provide for an easy succession into the job of postmaster general," said Michael F. Cavanagh, a private consultant who deals with postal issues. "The governors saw fit to change that. There's going to be a reevaluation of some of the programs and management styles."

The governors, said Cavanagh, "were saying Carlin wasn't the one to lead them into innovation and no one else at L'Enfant Plaza Postal Service headquarters was either."

Carlin did not return a reporter's telephone calls. A spokesman at the Postal Service said all questions would be answered when the board holds a news conference this morning.

While the timing of Carlin's firing was unexpected, board members were known to be disappointed with him as early as March, just three months into his tenure.

Van H. Seagraves, publisher of a trade newsletter called "Business Mailers Review," which has accurately predicted postal developments in the past, wrote in his March 18 edition that the governors were angry with Carlin over delivery standards.

"Paul Carlin, the new PMG, is on the spot," Seagraves wrote then. "Some governors are giving him a limited amount of time to get results."

Seagraves reported again in December that the governors were unhappy with Carlin for refusing to go along with plans for massive layoffs as part of the effort to modernize the mail monopoly and hold down its costs as it prepares for the next century. Seagraves reported then that unless Carlin "can regain in the next couple of months the confidence of his governors and senior managers, he can't survive."

The labor-intensive Postal Service, the largest U.S. agency, lost $251.5 million in the fiscal year that ended last September, and its work force grew from 702,000 in 1984 to 740,000 by midsummer 1985.

Board members were also critical of Carlin for allegedly failing to master the intricacies of some of the service's complex issues like the nine-digit zip code and the E-Com electronic mail venture.