He has published over 130 articles and his laboratory has elucidated the first high-resolution structures of a LacI, MerR, or MarR family members bound to cognate DNA, Hfq-RNA complexes, and the first multidrug-binding transcription regulators bound to multiple drugs.

Beyond his research, Dr. Brennan has a long-standing interest in graduate and postdoctoral education and training and has served on the NIGMS Biomedical Research and Research Training grant study section. In 2004, he was awarded the John A. Resko Faculty Research Achievement and Mentoring Award from the Oregon Health & Science University. He served as a counselor for the Biophysical Society and is a member of several professional societies, including the American Association for the Advancement of Science, American Crystallographic Society, American Society for Microbiology, American Society for Biochemistry and Molecular Biology, and the Biophysical Society. He is currently serving on the editorial boards of Molecular Microbiology, the Journal of Bacteriology and mBio. He was elected Fellow of the American Academy of Microbiology in 2007 and Fellow of the American Association for the Advancement of Science in 2011.