Jo Ann Gasper, a social-services official at the Health and Human Services Department, will succeed Marjory E. Mecklenburg, who resigned Tuesday as director of HHS' population affairs office after a departmental investigation of her travel expense accounts, department sources said yesterday.

Mecklenburg, who ran the population program and the department's adolescent pregnancy program, was a controversial figure. Before joining the department, she was president and cofounder of an antiabortion group, American Citizens Concerned for Life. In the department, she wrote the "squeal rule," eventually killed by the courts, that would have forced federally funded clinics to notify parents when minor daughters sought birth-control prescriptions.

Mecklenburg, in a letter of resignation to HHS Secretary Margaret M. Heckler, called allegations of wrongdoing "unfounded and unfair" and blamed some of her troubles on bureaucrats who did not want to carry out President Reagan's policies.

The investigation, conducted by the office of Inspector General Richard Kusserow, looked into 14 government-paid trips Mecklenburg took in 1983 to 1984 at a total cost of about $13,000.

During a trip to a conference in Denver last November, according to reports, she attended a Denver Broncos football game in which her son, Carl, was playing against the Minnesota Vikings. The investigation was revealed by Jack Anderson in his syndicated column.

The investigation, according to department sources, concluded that Mecklenburg was not guilty of intentional wrongdoing. The Justice Department concurred in that judgment.

In her letter, Mecklenburg told Heckler that the complaints "are nothing more than philosophical differences with this administration" by "those in the bureaucracy who are too inflexible to adapt to pursuing the goals of this administration. The dissenting bureaucrats have brought continuous pressure to bear, so that I have had to achieve my goals while dealing with contrived and baseless assaults."

Although the inspector general's report cleared Mecklenburg of criminal wrongdoing, department sources said conflicts over her "management style" and the trips had made it impossible for her to function effectively.

Antiabortion activists yesterday said they regretted her decision to resign. "We feel she has worked very effectively in a very difficult agency and situation," said Jan Carroll of the National Right to Life Committee.

But Susan Cohen of the Alan Guttmacher Institute, a leading organization in population issues, said her organization had had difficulties with family-planning programs under Mecklenburg. Cohen said she hoped Mecklenburg's successor "will operate the program in a way that is in accordance with the intent of Congress."

Department sources said Mecklenburg will serve for a time as consultant to its consumer affairs office before returning to Minnesota.

Gasper, Mecklenburg's successor, has been a deputy assistant secretary for social-services policy since November 1981. She is said to be a longtime supporter of Reagan's antiabortion policies.

Judie Brown, president of the American Life Lobby, said the choice of Gasper was "encouraging," but she added: "Frankly, I think she will have just as difficult a time making it as Marjory Mecklenburg did. Family planners will be her biggest problems."

Before joining the HHS, Gasper was a business consultant and publisher and editor of a newsletter called "The Right Woman."