A group of out-of-town investors has offered $9 million for M. S. Ginn and Company, Washington's biggest office supplies and services business, Ginns chairman Marsh S. Marshall announced yesterday.

The formal offer will be presented Monday to the board of directors of Ginns.

The offer amounts to about $12.25 a share for Ginns stock which has traded lately for about $8.75. At the company's request, over-the-counter trading in the stock was halted yesterday.

Marshall is the largest shareholder of Ginns, owning slightly more than half its common stock.

Another 20 to 30 percent is held by principals and customers of Ferris & Co., the local stockbrokers who sold the stock to the public in 1970.

George Ferris Jr., president of the brokerage house, said that including four 10 percent stock dividends, purchasers of the original offering would get about 3.5 times their initial investment at the $12.25 price.

Marshall said the Ginns board will decide Monday what form the takeover could take. A direct sale for cash would require Ginns' shareholders to pay heavy capital gains taxes on their profits.

Ferris said one technique under consideration involves selling the company's assets, then investing the money in an investment fund, enabling shareholders to avoid the heavy tax bite.

Ginns officials would not identify the source of the offer, but indicated it came through a New York investment banking firm.

Ginns has a net worth of about $6 million. Last year it earned just over $1 million on sales of $38.7 million.