More than 15 years after he left Harvard to pursue a successful career in New York City government. Robert F. Wagner, Jr., '65 will return to Cambridge to lecture on public policy and do some writing.

The four-year deputy mayor for policy and a powerful figure in New York government for 10 years. Wagner resigned last week to become one of six fellows at the Kennedy School of Government's Institute of Politics (IOP) for the spring semester.

The former Marshall Scholar said yesterday he had accepted the IOP offer as "a good break to put my public service career in perspective and think about the future." Wagner will be the 12th deputy mayor to leave the Koch Administration.

Last month, David R. Gergen, assistant to President Reagan for communications, also resigned his post to take the six-month IOP fellowship.

In the wake of the fiscal crisis of 1975, Wagner emphasized rebuilding the city's crumbling infrastructure rather than beginning new construction. The 39-year-old administrator is also known in government circles for his dedication to New York's health and education services, colleagues said.

"He helped to build a new spirit of cooperation between City Hall and the Board of Education over the budget," says Leonard Supp, Wagner's executive assistant for eight years, Supp added that as chairman of the Health and Hospitals Corporation. Wagner was also able to "turn the Corporation around" from deficits to a firm financial footing.

Accepting the resignation of his chief policy advisor, Mayor Koch said last week that Wagner "brought to this city a rare insight and historical perspective."

Wagner, the son of the former Mayor, suffered what colleagues say was a "great disappointment" earlier this year when his coveted appointment as Schools Chancellor was overturned by State Education Commissioner Gordon M. Ambach. Ambach said at the time that the deputy mayor was "technically unqualified" for the post.

But Wagner says that his failure to secure the appointment was in no way a factor in his decision to resign from city government. According to his long-time friend and campaign manager John Cownorton, the deputy mayor had been anxious to leave city government "for a respite in academic world" a year ago, but the possibility of the new post made him delay his resignation.

"He stayed as long as he did out of loyalty to Koch," Cownorton says. Wagner lasted longer than any of his predecessors in the job, known for its high rate of turnover.

"Being deputy mayor isn't a job one holds for a lifetime," agrees Supp. "I think Bob knew it was the right time to make a change."

As an IOP fellow next semester. Wagner says he will teach a study group on the urban issues of the next decade, and do independent writing on "America's most urgent domestic issue," urban poverty.

He says he also plans to consult for the 20th Century Foundation, a group which funds public policy studies. He will continue to serve on the board of New York's Metropolitan Transit Authority, overseeing a five-year capital program he helped put together, and will head a recently-announced commission on "New York in the year 2000."

"I'm a glutton for punishment," Wagner explains.

The IOP chooses fellows for their significant public service or journalistic experience, says Fellows Coordinator Theresa Donovan. Wagner, who plans to live off campus, will receive the normal monthly fellow stipend of $2000, Donovan added.