Robert Giffin, the protege of House Speaker Thomas P. O'Neill D-Mass.), who spent 30 years working his way into the No. 2 job in the agency that buys almost everything the federal government needs, has taken his first job in private industry.

Griffin has gone to work for Chrysler Corp., running the Washington office that sells cars and trucks to the government.

Griffin heads Chrysler's Government Sales Division, which last year did an estimated $75 million worth of business with Griffin's former employer, the General Services Administration.

Griffin is the largest supplier of cars and trucks to GSA, which buys vehicles for every agency except the armed forces. Last year GSA bought 50,000 cars and trucks worth $300 million, and Chrsler got 26 percent of the business.

Griffin was fired as deputy administrator of GSA in 1978 by his boss, Jay Solomon. Solomon claimed Griffin blocked investigations of corruption in the federal housekeeping agency and sabotaged efforts to reorganize GSA's bureaucracy.

But because of his connections to O'Neill, Griffin's firing got Solomon in more trouble than it did Griffin.

After the powerful house speaker raked White House aides over the coals for letting his friend get fired, President Carter personally praised Griffin and found him a $50,000-a-year job on the staff of Special Trade Representative Robert Strauss.

O'neill recently had the government trade his Lincoln Continental limousine for a Chrysler.

Solomon was not the first to try to oust Griffin, who built a power base that permeated GSA. Solomon's predecessor, Jack Eckerd, objected to appointment of Griffin as his right-hand man and finally quit rather than work with him.

Chrysler officials confirmed that Griffin has been hired for the government sales job, but shielded him from inquiries. "Mr. Griffin does not talk to reporters," his secretary said yesterday, referring the caller to a Chrysler corporate public relations person in Michigan.

Apparently neither GSA nor the trade representative's office have any ethics code that prohibits Griffin from moving from the buyer's side of the bargaining table to the seller's side.

Virtually all of GSA's cars are purchased by competitive bidding. GSA officials said the government is the only auto buyer that deals directly with the manufacturer rather than through dealers. As a result of direct purchases and its large volume, the government buys cars for less than dealers pay for them, GSA officials said.

Chrysler has been the leading supplier of government civilian vehicles for at least the past four years, GSA statistics show. In fiscal year 1977, Chrysler got 26 percent of the government car and light truck business; in 1978, 34 percent; in 1979, 26 percent and about the same percentage in 1980, agency officials said.

Griffin's job does not include Chrysler's sales of the controversial XM-1 tank to the Army or any other military sales by the company, a Chrysler spokesman said. Nor will the Washington veteran be involved in the lobbying for government aid to Chrysler, he added.