[134] In this landmark case, Ex parte Milligan, Garfield successfully argued that civilians could not be tried before military tribunals, despite a declaration of martial law, as long as civil courts were still operating. In his very first court appearance, Garfield's oral argument lasted over two hours, and though his wealthy clients refused to pay him, he had established himself as a preeminent lawyer.[133]
During Grant's first term, Garfield was discontented with public service and in 1872 again pursued opportunities in the law. But he declined a partnership offer from a Cleveland law firm when told his prospective partner was of "intemperate and licentious" reputation.[135] In 1873, after Chase's death, Garfield appealed to Grant to appoint Justice Noah H. Swayne Chief Justice, but Grant appointed Morrison R. Waite.[136]

Garfield's proof of the Pythagorean theorem features a right triangle within a trapezoid.
In 1871, Garfield traveled to Montana Territory to negotiate the removal of the Bitterroot Salish tribe to the Flathead Indian Reservation.[137] Having been told that the people would happily move, Garfield expected an easy task. Instead, he found the Salish determined to stay in their Bitterroot Valley homeland. 