Born in Chicago, Illinois, Ryan graduated from Homewood-Flossmoor High School, then Knox College with her Bachelor of Arts degree in 1985. Ryan attended law school under the Marine Corps Law Education Program at the University of Notre Dame Law School, where she also was a member of the Notre Dame Law Review.
Following law school graduation, Ryan served on active duty for the United States Marine Corps from 1988–1992, and again as a judge advocate from 1995-1999. Ryan served in units within the II & III Marine Expeditionary Forces as a Staff Officer, Company Commander, Platoon Commander, and Operations Officer. Judge Ryan's tours included deployments to the Philippines, during a coup attempt, and to Saudi Arabia during Desert Shield and Desert Storm. As a Judge Advocate General (JAG) officer, Ryan served as a Trial Counsel and Chief Trial Counsel in Okinawa, Japan and Quantico, Virginia. Ryan was then selected by General Charles C. Krulak, Commandant of the Marine Corps, to serve as his Aide de Camp.
Ryan was law clerk to Judge Michael Luttig on the United States Court of Appeals for the Fourth Circuit, and then to Justice Clarence Thomas on the United States Supreme Court in 2001-2002.
Prior to joining the court, Ryan was in private practice with various firms. Most recently, she was at Wiley Rein LLP from 2004 until her appointment to the court. Before that she was with Bartlit Beck Herman Palenchar & Scott from 2002–2004 and Cooper, Carvin & Rosenthal from 1999-2000.
Ryan was nominated by President George W. Bush on November 15, 2006 to replace Judge H. F. Gierke III, who retired September 30, 2006. She was confirmed less than a month later by the U.S. Senate on December 9, 2006 by unanimous consent during the closing hours of the 109th Congress. Ryan's appointed for 15-year term is due to expire on July 31, 2021.
In 2012, Judge Ryan joined the court majority when it found that it did not have jurisdiction to order disclosure of trial documents from the Chelsea Manning court-martial. When the court majority reversed the conviction of a soldier for attempting suicide, Judge Ryan dissented, arguing that the appeals court did not have jurisdiction.
In September 2016, Ryan was named as a possible nominee to the Supreme Court of the United States by Republican presidential candidate Donald Trump.
Judge Ryan is married to Michael J. Collins.