In 1983, he became a research fellow at the Howard Hughes Medical Institute's Laboratory of Molecular Endocrinology, studying transcriptional regulation in endocrine genes with a focus on nuclear hormone receptors.

[5] During this period, he served as chief of the Thyroid Unit at Massachusetts General Hospital from 1987 to 1993, where he co-directed training programs in endocrinology, diabetes, and reproductive biology.

He directed Northwestern’s Training Program in Endocrinology and Metabolism, focusing on endocrine gene transcription, thyroid disease, and pituitary tumor pathogenesis.

[6][5] In 2000, Jameson became chairman of the Department of Medicine at Northwestern University, overseeing a broad range of research initiatives and patient care services across multiple specialties.

That same year, he also joined Harrison’s Principles of Internal Medicine as an editor, contributing to its 15th through 21st editions and authoring sections on endocrinology, genetics, and sex development.

In December 2023, Jameson was appointed interim president of the University of Pennsylvania, following the resignation of Liz Magill amid her congressional hearing controversies.