Ehrenfeld's research interests include bioinformatics and the application of information technology to increase quality, reliability and patient safety.

His appointments have included Assistant Professorships at Harvard Medical School and the Massachusetts General Hospital where he practiced anesthesiology within the Department of Anesthesia, Critical Care, and Pain Medicine.

He was later assigned to Walter Reed National Military Medical Center, and served with an Operational Health Support Unit in Nashville, Tennessee.

In 2014, Ehrenfeld was called to active duty and served a tour in Kandahar, Afghanistan at the NATO Role III Multinational Medical Unit.

[24][25] A longtime advocate for patients, equality, and LGBTQ health, Ehrenfeld made international headlines on February 22, 2015, when he asked the newly appointed Secretary of Defense Ashton Carter his stance on letting transgender individuals serve in the military.

The Secretary's response "I don't think anything but their suitability for service should preclude them [from serving]"[26] — was his first public comment on the issue and the most favorable from a senior U.S. military official to date.

Within hours, the event was being reported by news outlets all over the world and by the next afternoon the White House chimed in with its support, sparking even more media attention.

[37][38][39] In his testimony, Ehrenfeld said "I would like to state unequivocally that there is no medically valid reason—including a diagnosis of gender dysphoria—to exclude transgender individuals from military service.