[3] Wallach joined the firm of Lionel Sawyer & Collins in Las Vegas, Nevada, as an associate, in 1976.

[3] He attended the Judge Advocate General's Legal Center and School at the University of Virginia and served in the United States Army Judge Advocate General's Corps in the International Affairs Division of the Office of TJAG at The Pentagon during the Gulf War, where he assisted in advising on the law of war and investigating war crimes allegedly committed by Iraqi leaders.

From 1989 to 1995 he served as Judge Advocate General in the Nevada Army National Guard, with the rank of major.

His responsibilities included giving annual lectures to Military Police regarding their legal obligations on treatment of prisoners.

Wallach is a member of the International Law of War Association, which is a "loose confederation of military lawyers, academics, and government officials including members of the judiciary, who are interested in the advancement of a legal regime to ameliorate suffering and for the regulation of the use of armed force in armed conflicts".

[3] President Barack Obama nominated him on July 28, 2011, to serve on the United States Court of Appeals for the Federal Circuit.