Final answer:
John Marshall's role in the new American government included serving as Secretary of State and most notably as Chief Justice of the Supreme Court, where he established the precedent of judicial review and significantly impacted the federal legal system.
Step-by-step explanation:
John Marshall played a pivotal role in the new American government, serving first as the Secretary of State under President John Adams in 1800. However, his most significant contribution came when he was appointed as the Chief Justice of the United States in 1801, a position he held for thirty-four years. John Marshall is renowned for his role in establishing the practice of judicial review with the landmark decision in Marbury v. Madison (1803), whereby the Supreme Court asserted its authority to invalidate congressional acts that were in conflict with the Constitution. This bold move cemented the Supreme Court's role as a coequal branch of government and significantly shaped the course of American law and governance. As Chief Justice, Marshall worked towards fostering the Supreme Court's role in guiding national policy and public opinion, often supporting the Federalist party's nationalist agenda. His tenure is frequently credited with defining the modern court's role in the U.S. governmental system and strengthening the judicial branch.