Helen Leland Witmer was born on July 17, 1898, in Lansford, Pennsylvania, to parents I. K. and Nellie (Seager) Witmer, and raised in Lancaster, Pennsylvania. She began her comprehensive education at Dickinson College in Carlisle, Pennsylvania; receiving her B.A. After working for a couple of years as a high school teacher in Reading and Lancaster, Witmer then continued her education by earning her Master's and Ph.D. from the University of Wisconsin, in between earning her certificate in social work from Bryn Mawr College in Bryn Mawr, Pennsylvania.

In 1925, the year she earned her Master's, Witmer began what would be a significant career in children's welfare as a statistician in Massachusetts' Division of Corrections. Soon after, however, in 1926, she took a job at the University of Minnesota in Minneapolis as an assistant research professor of sociology. She held this position for a year, then concluded her education with a post-doctoral study at the London School of Economics and Political Science from 1927 to 1929.

After rounding out her education, Witmer became director of research at Smith College's School of Social Work in Northampton, Massachusetts, in 1929. Witmer stayed at Smith for 20 years, also working as editor of the school's publication, Smith College Studies in Social Work. Her next job was at the University of California in Los Angeles, where she was professor of social welfare from 1949 to 1951. During her time at UCLA, Witmer was also the director of fact-finding for the White House Conference on Children and Youth. Like the heart of Witmer's research, the conference's architects described its purpose as: "To consider how we can develop in children the mental, emotional and spiritual qualities essential to individual happiness and responsible citizenship and what physical, economic and social conditions are deemed necessary to this development," according to Dean W. Roberts, a representative of the American Public Health Association at the time.

Witmer then left UCLA to focus her work in children's welfare completely within the government. In 1951, she became director of the Division of Research for the U.S. Children's Bureau in Washington, DC. Witmer's responsibility at the Bureau, where she would remain until her retirement in 1967, was to devise research standards for evaluating federally funded children's programs. Afterwards, she worked as editor of the Bureau's publication, Children's Bureau Research Reports.

Throughout Witmer's career, her research led her to publish articles and books on many aspects of children's welfare; particularly juvenile delinquency, pediatric psychiatry, family welfare, daycare, parent education, adoption and child guidance. Child guidance; a movement initiated by the Commonwealth Fund, a foundation that aims to improve health care for especially vulnerable Americans; was just gaining its momentum in the early 1900s, and was therefore a major focus of Witmer's research. It focused on mental and behavioral issues in children; finding ways for them, their families, and mental health professionals to cope with their problems.

In her first book primarily focused on social work, Social Work: An Analysis of a Social Institution, Witmer examined the nature of social work and its function, including why social work needs to exist as an institution; its basic principles; and how it is divided into fields, with each field carrying out a different function of the institution. In the preface, Witmer explained that while she originally intended for the book to be a text for undergraduate students, once she began writing she realized that she herself still had questions about the definition and limitations of social work. "This book in consequence records the development of an idea," she explained, "an idea regarding the nature and function of social work. That it begins by posing a question and proceeds to successively closer approximations to an answer is, accordingly, no mere pedagogical device but a serious record of an attempt to discover what social work really is."

The adoption advocate later went on to co-author a study on adoption, called Independent Adoptions: A Follow-up Study. As cited in the book's introduction, in the year it was published, 1963, adoption was "among the hotly debated issues of public policy relating to children." In preceding years, adoptions had been handled mainly by special federal agencies. But in the 1950s, "independent" adoptions were becoming more popular; handled by the adopted child's parents themselves, or intermediaries such as lawyers. According to historian Ellen Herman, the study was "...the most ambitious of all outcome studies at mid-century... the premise was that independent adoptions were more likely to fail than agency adoptions because they lacked professional expertise and took shortcuts around regulatory safeguards." Surprisingly, however, the study, sponsored in part by the Florida Department of Public Welfare and the U.S. Children's Bureau, did not support this hypothesis. Rather, says Herman, "The widely publicized view that independent adoptions were dangerous was obviously exaggerated, if not incorrect." Witmer had unexpectedly paved the way for independent adoption to flourish in the years to come.

After years of making invaluable contributions to the institution of children's welfare, Witmer died of a brain tumor on November 20, 1979, at the age of 81.