Zack is a former president of the American Bar Association and was the first Hispanic American to assume the ABA presidency. He was nominated by President Barack Obama for the position of Alternate U.S. Representative to the Sixty-eighth Session of the General Assembly of the United Nations and also served as Senior Advisor to the U.S. State Department.

Zack was the first Hispanic American and youngest President of the Florida Bar. He has also served as president of the National Conference of Bar Presidents and as the chair of the ABA's House of Delegates. He is a member of the ABA International Law Section and has served on the ABA Board of Governors.

He served as general counsel to Florida's former Governor and U.S. Senator Bob Graham and has represented the Florida Senate; the Florida Department of Professional Regulation; the Cities of Hollywood, Hialeah, Miami Beach, Miami and numerous other national clients such as National Geographic Society. In 2000, he represented former Vice President Al Gore in the trial of Bush v. Gore.

Among many other accomplishments, Zack has served as Chair of the Florida Ethics Committee and was appointed by Governor Lawton Chiles to serve as a member of the Florida Constitution Revision Commission. In November 2009, Zack was selected by the University of Florida to receive the distinguished Alumnus Award.