Sleepy-eyed and slow of speech, Rep. George H. Mahon (D-Tex.), chairman of the House Appropriations Committee, always looked like an easy mark to young liberal newcomers to Congress, who thought his fiscal conservatism and his hawkish views on defense out of date.

It was a mistake. As someone once said of the gentlemanly, almost courtly, Mahon, "He never argues with anybody and he never loses an argument." A liberal member of his committee once described taking on Mahon as "like punching a pillow. He gives and gives, but when the votes are counted, he's still in good shape."

When Congress meets again in January, Mahon won't be back. At the age of 78, he is ending the longest term of consecutive service of any current member of Congress, 43, years. Along with a record-breaking 56 other senators and representatives, Mahon is retiring this year.

Retirements have been running at a record high since 1972, partly because already lucrative pensions were increased, and partly because increased legislative and constituent service loads have made the job more time-consuming and burdensome, and public cynicism has made it less enjoyable.

This year's retirements run the gamut from those who made an impact to those who were barely heard from. Some are candidates for other office. Some have had age or health problems catch up with them. Some, like Reps. Michael Harrington (D-Mass.) and Gary A. Myers (R-Pa.), are young and healthy, but disillusioned with Congress. Some mark the end of an era.

Mahon's retirement is notable, not only because someone who had almost become a fixture is leaving but also because he is the last of a breed in the House - the strong chairman who tightly controlled his committee and was almost invincible on the floor.

Most of that breed were swept out in the coup of 1975, the culmination of a revolt against autocratic chairmen and the seniority system that saw three Old Guard types deposed as chairmen.

Mahon survived because he was considered eminently fair, and was flexible enough to accept just enough change to keep the reformers at bay. He lot on issues, of course - funding the Vietnam War was one - but he never lost control.

When reformers were out of depose one of his subcommittee chairmen, Jamie Whitten (D-Miss.), Mahon simply removed the irritating factor. He took consumer and environmental jurisdiction away from Whitten, who angered liberals by his attacks on those agencies, and gave it to another subcommittee. Whitten survived, and is now likely to be Mahon's successor as Appropriations Committee chairman.

On the Senate side, there is a comparable "end of an era" story in the retirement of Sen. James O. Eastland (D-Miss.). Eastland, a 37-year verteran, is among the last of the Dixiecrats, the Southern segregationists. With his leaving, the Senate Judiciary Committee will pass into the hands of Sen. Edward M. Kennedy (D-Mass.), and the days when civil rights bills were bottled up in the committee will fade into memory as will Eastland's speeches on "mongrelization of the races" or the "communistic" Supreme Court.

With Eastland goes the last Dixiecrat leader in the House, Rep. Joe Waggonner (D-La.), who in recent years had turned his attention to the Ways and Means Committee on which he served. Waggoner became a black hat to a new group, the public-advocacy lobbies, as he sought, and often got, tax breaks for big business, oil and gas interests.

As an indication of how much things have changed, the first black in Mahon's Texas delegation - Rep. Barbara Jordan (D), is also leaving. First elected in 1972, Jordan, 42, was not known for her impact on legislation, but her impact in debate; her clipped accent and powerful voice made it sound "as if God were delivering the Ten Commandments," as one of her colleagues used to say.

The country came to know her speaking style during the Nixon impeachment inquiry of the House Judiciary Committee and later during a rafter-raising speech at the 1976 Democratic National Convention.

Though she was a staunch supporter of civil rights and women's rights, Jordan was often jokingly referred to as a "closet conservative" because the House members who clustered around her back row seat were invariably conservative Democrats and right-wing Republicans.

The House is also losing Richard Nixon's ablest defender during the impeachment inquiry, Rep. Charles Wiggins (R-Calif.), the man whose insistence on specific proof caused the Democrats to retreat until they could come up with blow-by-blow indictments.It was also Wiggins' desertion of Nixon when the tape that contained the "smoking gun" was revealed that started Nixon on the road to resignation. A strict interpreter of the Constitution, Wiggins was also a master debater, whose arguments could change the outcome of a vote.

The Senate is losing its main maverick, Sen. James Abourezk (D-S.D.), who often, tilted at windmills in opposing what he considered giveaways to the oil industry and demanding greater protection for the rights of Indians and consumers, is quitting with few regrets.

I can't wait to get out of this chicken - outfit" Abourezk said of the Senate during the closing days of the recent session. He ended his career right in character, unsuccessfully filibustering against the energy bill, a measure he considered a ripoff of the American consumer.

On the House aide, an Abourezk without humor is quitting. Rep. Harrington is also leaving the way he began - with a blast at the system and the way Congress does business.

Harrington, a fervent liberal, crusaded to make known and stop Central Intelligence Agency involvement in coups and assassinations attempts . He formed the Northeast-Midwest coalition to attempt to stem the flow of federal dollars to the Sun Belt. And he raged even against his younger colleagues in Congress, calling them sterile managers and adding he would prefer the oldtime villains who believed in something.

Unlike Harrington, another retiring maverick Rep. Otis Pike (D-N.Y.), always unleashed his barbs with a witty point. Pike may be best remembered for taking to the House floor to oppose giving flight pay to deskbound Pentagon generals. Pike said nothing about the bill. He merely described the hazards of flying a desk.

In the Senate, long-time establishment member John J. Sparkman (D-Ala.) is leaving Sen Sparkman, the vice presidential candidate on the 1952 ticket with Adlai E. Stevenson wound up as chairman of the Senate Foreign Relations Committee, and presided over its transition from a hotbed of opposition to the Vietnam War under former Senator J. W. Fulbright to a more docile supporter of the president, as in the recent sale of planes to Saudi Arabia and Egypt.

The House is losing its crusading investigator, Rep. John Moss (D-Calif.) whose bulldog attacks have covered everything from drug pricing to the Civil Service Commissions. On the same committee with Moss, Commerce, the man known as "Mr. Health," Rep. Paul Rogers (D-Fla.), is leaving. Rogers battled for clean air laws, hospital cost containment and unsuccessfully, national health insurance.

And the House Education and Labor Committee is losing the man known as "Mr. Education." Rep. Al Quie (R-Minn.), though he was on the minority side, is credited with being more knowledgeable and having more influence on education bills than any man now in Congress.

Finally, the Senate is losing Sen. William Scott (R-Va.), who distinguished himself by the number of junkets he took and by being named "King of Dumb," the dumbest man in Congress, by New Times magazine. Scott responded by calling a press conference to deny he was dumb.