In the 16 years he has helped manage the government's mass transportation programs, Robert H. McManus has worked under five presidents and so many UrbanMass Transportation Administration bosses that the career senior executive feels "like a pony express rider. I'm just changing horses."

Under the Reagan administration, the 56-year-old former city manager is helping to dismantle or curtail some of the mass transit programs he once worked to design --programs set up to distribute federal construction and operating funds to traffic-snarled urban corridors. But where the Carter people worked to generate new mass transit systems, the Reagan people have halted funds for new construction, at least until the economy is in better shape, and plan to phase out operating subsidies.

"That's a definite turnaround," said McManus, UMTA's associate director for planning management and demonstrations. Like a career diplomat, however, adjusting to a new set of policy makers is all part of his job. The consummate bureaucrat, he prides himself on being the type of professional who, without being a yes man, can help political appointees make their programs work.

"It's simplistic to think that the federal bureaucrat just carries out what someone tells him to do," McManus said recently. "I have strong opinions, and I express them. But you try to quell your rhetoric about elements of the program that are not in vogue with the current administration and look for elements you can push forward."

McManus gets high marks from previous Republican and Democratic administrators for his usefulness in such sensitive political changeovers. Indeed, he has played the role of UMTA transition expert so often and so well that one Department of Transportation official notes admiringly, "You can change the heads of the agency, but it's people like him who keep it going."

But McManus is more than a survivor. He has held virtually all the key management posts in the agency at one time or another, including two stints as its acting director. "He's a person able to understand both sides of the issue and give his expert advice," said Arthur Teele, UMTA's current administrator and McManus' 10th boss.

Under Reagan, however, Teele admits that McManus "has lost a lot more arguments than he has won with us," and his opinions have rankled some Republican appointees. But Teele says he trusts McManus to carry out the administration's policies, "and I tell them critics that if they don't want to know what Bob thinks, don't ask him."

McManus objected, for example, when the administration decided to withhold federal funds from cities that were studying their various transportation options, including possible rail construction. McManus argued, unsuccessfully, that the technical integrity of transportation planning made it necessary to consider all forms.

"If we're saying we're not stopping federal funding of rail construction forever, then why can't we at least get the projects planned?" he reasons.

He lost another in-house argument over the administration's decision to scrap $100 million worth of Carter-approved grants designed to encourage commercial development around transit stations. "I wanted the incoming administration to get to its bottom line more gracefully . . . to play out the grants already made and then eliminate the category," he explained.

Some of the disagreements are minor but no less frustrating. A productivity study involving three AFL-CIO unions has been under review ever since Reagan came under attack by labor leaders. A $25 million demonstration project to test a high-speed boat transit system between New York and New Jersey has been scaled down to a $4 million study of water transport systems worldwide.

"We spent two years working out the details of the speedboat project, and just when we're ready to go, there's a change in administration," McManus said.

But despite such setbacks, "life goes on" at UMTA. Right now, McManus is heading up a special task force appointed by Teele to evaluate the agency's mission -- "what business we're in and what business we're not in" --under the philosophical and budgetary restraints of the new order.

McManus' office still administers a lot of planning grants and research and demonstration projects, but UMTA, for the first time in its 13-year history, has begun to scale back its construction activities, concentrate on current operations, and try to make the best of what it has.

"I'm not an idealogue, except in saying that the government should support mass transit as a worthy purpose," McManus said. "I play a long game and a short game, and I'm pretty stubborn. But I don't offend the people I work with. I know that you have to see these things play out over a long period of time."

On a recent workday, for instance, he reviewed and suggested revisions for a UMTA report being sent to Congress, worried over the gentlest way to tell members of an advisory committee on transit statistics that the new administration wants to pick its own people, and delved into the complexities of UMTA's new transit funding policy.

"I don't have a typical day," McManus said.

He can be conferring about transit financing with members of the British Parliament one moment, meeting with a congressional oversight committee staff the next and, at other times, be "just swimming in paper," like any other government bureaucrat.

Transit issues clearly are McManus' passion. Where another's eyes might glaze over at discussions of rail project evaluations, maintenance improvements and engineering surveys, he warms to such topics. The performance problems of bus systems are his problems, and the productive uses of highway and transit investments -- including innovative experiments with van pooling, dial-a-ride or shared-ride taxi services -- are some of his pet concerns, particularly if they work to reduce downtown traffic congestion.

"You can't justify mass transit on the basis of energy savings or air pollution," McManus said. "But it is a social purpose for the people who don't have any other way to get around: the poor, the handicapped and people who are temporarily unable to get a driver's license."

Reaganites, he acknowledged, "really don't have bleeding hearts" on some of these issues. "Their driving objective is the economy . . .but you don't have to have a bleeding heart to see the program as an essential service for the vitality of the cities."

When Frank Herringer, a UMTA administrator under presidents Nixon and Ford, showed up at the agency in 1972, McManus stood out from the beginning.

"I was 30 years old and had no essential background at all in transportation, and I quickly looked around my first day and picked out Bob as one of the key people I could rely on," said Herringer, now an executive with the Transamerica Corp. in San Francisco.

Richard Page, general manager of the Washington Metropolitan Area Transit Authority and a UMTA administrator under former president Carter, says that under the British system of government McManus would be considered the "permanent undersecretary" of the agency.

Page was present last February when McManus had to address a conference of mayors and other city officials, and recalls: "He was a career man sent out to make a statement in a highly political situation, and he did a good job of putting the best face on brand-new policies that were a 180-degree change from what he had been working under only 60 days earlier."

McManus found out all about the need for smooth transitions during the 17 years he spent in municipal management. In his hometown of Hartford, in Rome, N.Y., and in Fond du Lac, Wis., where he was city manager for seven years, he got a close-up view of local governments and developed a keen sensitivity to their concerns about bureaucratic red tape. He also became accustomed to the turnover of political authorities, with city councils sometimes changing every one or two years.

McManus is a political science graduate of Wesleyan University, with a master's degree in public administration from Syracuse University. He joined the federal government in 1965, moving to UMTA when it was created three years later. This year he was in the first group of senior executives to attain the special Presidential Rank of Meritorious Executive, an honor for superior accomplishment in government.

The $10,000 award that came with it was also appreciated: McManus is putting the last two of his five children through college on a salary that, like other senior executives, is capped at slightly over $50,000.

At work most mornings by 8:30 a.m., commuting by bus and subway from his Potomac home, McManus spends most of his time in a roomy, cheerfully paneled office overlooking L'Enfant Plaza and the Southwest Freeway. Several plaques and citations are hung casually on the walls. His desk is a long wooden conference table. Most days he works exclusively from this room, supervising a staff of 75 and a budget of $65 million, and doesn't leave the office until after 6 p.m.

"When I first came to Washington, I thought I'd go up the wall with all the paper pushing you have to do before you get things done," he said. "I was used to being the boss." But he adds that once he had made contacts in Congress and in the agency, "I got to see how you shrink this bureaucracy by knowing the right people, and I got to be more effective."

McManus has learned firsthand how the varying backgrounds and interests of different agency administrators can affect UMTA priorities.

John Volpe, for instance, was a contractor, and as Nixon's transportation secretary was "all hot to trot with technology," recalled McManus. But the UMTA administrator in Nixon's second term "was an economist, and he could care less about modern technology."

And when Neil Goldschmidt, Carter's last DOT chief, touted mass transit as a cure for air pollution, McManus' office knew the problem had more to do with the way cars are manufactured. Transportation secretaries "are a bit more vocal in their exaggerations -- Volpe was incorrigible -- but I kept his administrator careful."

But McManus has never had the kind of disagreements that have made him think of quitting. He concedes that some of the transit areas he is interested in "are very touchy subjects" with the Reagan administration, but says that its central concerns, such as management efficiency, are his concerns, too.

Unless the UMTA program is "wiped out lock, stock and barrel," he said, "I can find creative ways to function."