He is notable for questioning whether the Department of Justice and Department of Defense should rely on paid witnesses when trying to assemble a new case against his client, Mohammed Jawad. His work on the case was featured in the February 2011 issue of GQ magazine.
Montalvo is Founding Partner of The Federal Practice Group,  a team of nationally recognized federal practitioners focused on the unique bodies of law that the U.S. federal government has created. His areas of practice are military law, security clearance law, international corporate law, and white collar crime.
On July 22, 2009 US District Court Judge Ellen Huvelle granted Jawad's habeas corpus petition, ruling that all the evidence against him was inadmissible, because it was triggered by torture. The Department of Justice requested time to lay new charges against Jawad, based on witness statements that had not been available during the previous five years he had been in custody. On August 4, 2009 the Washington Independent reported that Montalvo and lead counsel David Frakt reported that the new prosecution witnesses had received gifts or cash in return for their testimony. Montalvo traveled to Afghanistan to interview the witnesses.
On August 28, 2009, Jeremy Page, writing in The Times, reported that Montalvo was planning to represent Jawad in a lawsuit against the US government. Page reported that Montalvo was scheduled to retire from the Marine Corps within the next month.
On September 21, 2009 historian Andy Worthington, author of The Guantanamo Files, published a letter from Montalvo's colleague Frakt, that explained Montalvo's role in more detail. He wrote that, initially, Jawad's Defense team was going to hire a private investigator to travel to Afghanistan to conduct their own investigation, because so much of the evidence in the case had disappeared. It was only when Susan Crawford declined to budget for a private investigator that Montalvo made plans to serve as the team's investigator.
Defense of Adam Winfield
Adam Winfield is a soldier who faces charges that he participated in a thrill kill murder ring in Afghanistan. Montalvo is serving as his attorney. Montalvo and Winfield's father have asserted that Winfield was not a willing participant in the conspiracy, that he was one of the whistleblowers who had tried to report the ring.
- Daphne Eviatar (2009-08-04). "Military Lawyer Claims U.S. Paid Gitmo Prosecution Witnesses: Defense Attorneys Say Afghan Eyewitnesses Received Cash or Gifts From the U.S. Government". Washington Independent. Archived from the original on 2009-08-04. Retrieved 2009-08-04.
- Daphne Eviatar (2009-07-31). "In Jawad Case, Both Evidence and Crime Remain Unclear". Washington Independent. Archived from the original on 2009-08-04. Retrieved 2009-08-04.
- Daphne Eviatar (2009-08-04). "Lead Military Lawyer Confirms Afghan Witnesses Said They Were Paid By U.S.". Washington Independent. Archived from the original on 2009-08-04. Retrieved 2009-08-04.
- "Eric Montalvo , ESQ". Fed Attorney. Archived from the original on 2009-09-03.
- Jeremy Page (2009-08-28). "Mohammed Jawad to sue US Government over seven years in Guantánamo". The Times. Archived from the original on 2009-08-03. Retrieved 2009-08-02. "Mr Montalvo was originally appointed to represent Mr Jawad in Guantánamo, but is now acting in a private capacity before his retirement from the US Marine Judge Advocate General’s Corps this month."
- Andy Worthington (2009-09-21). "The Military Lawyers Who Helped Free One Of Guantanamo’s Youngest Detainees". The Public Record. Archived from the original on 2009-09-21.
- Charley Keyes (2010-10-04). "Whistle-blowing soldier moved to solitary confinement". CNN. Retrieved 2010-10-10. "Winfield contacted his father about the serial killing ring inside his Stryker Brigade squad, and his father, Christopher Winfield, telephoned the Army but with little result, Montalvo said."
- "Army may have known in February about Afghan murder". CNN. 2010-09-28. Retrieved 2010-10-10. "After the killing of Afghan civilian Gul Mudin in January, Winfield called his father -- himself a retired soldier -- and told him what had happened, according to Adam Winfield's attorney, Eric Montalvo."