American lawyer best known for defending Supreme Court Justice Samuel Chase (Chase, Samuel) at his impeachment trial and Aaron Burr (Burr, Aaron) at his treason trial and for arguing the losing side in McCulloch (McCulloch v. Maryland) v. Maryland.

Martin graduated with honours in 1766 from the College of New Jersey (now Princeton University). For the next three years he taught school in Maryland. Moving to Virginia in 1770, he became superintendent of a grammar school and studied law and in 1771 was accepted into the Virginia bar. He moved back to Maryland shortly thereafter and enjoyed a lucrative practice there.

A patriot in the years preceding the American Revolution, Martin became attorney general of Maryland in 1778 and vigorously prosecuted loyalists (loyalist). He was a member of Congress in 1785, and in 1787 he served as a delegate to the Constitutional Convention. A strong anti-Federalist opposed to the plan for a strong central government, Martin displayed his disapproval of what the Convention produced by walking out without signing the Constitution (Constitution of the United States of America). Throughout the following year he fought in vain to prevent Maryland from ratifying.

Martin defended Associate Justice Chase (Chase, Samuel) in 1804, saving the Federalist judge from conviction on impeachment charges. In 1807 Martin came to the aid of Burr (Burr, Aaron), the former vice president, on trial for treason following his mysterious adventure down the Mississippi River.

Having resigned as Maryland attorney general in 1805, Martin took up that position once again in 1818 after having been a judge from 1813 to 1816. As the state attorney general he argued Maryland's right to tax the Bank of the United States in Maryland (McCulloch v. Maryland) (1819). He lost the case, a landmark decision in the contest between federal authority and states' rights.

Martin suffered a stroke in 1820. He resigned his office in 1822 and—impoverished and broken both mentally and physically—lived out his final years in Burr's New York home.