[5] Hallman began his legal career as an associate at the law firm of Brown and Bain, where he worked in the fields of domestic and international finance and taxation, intellectual property and anti-trust.

As an economist, Hallman has worked with colleagues at Stanford, Harvard, MIT, and the University of Chicago in analyzing markets and competition in various industries.

[2] In 2009 Hallman put his law practice on hiatus to become headmaster of Tempe Preparatory Academy, an independent public charter school where he had served as general counsel.

As a pro-business and pro-development mayor who frequently crossed the aisle to achieve his goals, the Arizona Republic referred to him as "one of those rare public officials who meticulously connects the dots".

During his second term he also served as Chairman of the Maricopa Association of Governments, and has been an ardent champion of cooperative development and improved international trade with Mexico given his long experience in the field.

[5] Hallman has been involved with numerous local charities such as East Valley Habitat for Humanity, Assistance for Independent Living, Tempe Community Action Agency, and the Boy Scouts of America.

He was the president of the Rio Salado Foundation and spearheaded efforts to preserve Justice Sandra Day O'Connor's former Phoenix residence, which was moved to the Arizona Historical Society grounds and now serves as a center for national and international problem solving and policy initiatives.