Final answer:
The leader of the Rough Riders during the Spanish-American War who later became president was Theodore Roosevelt. Known for his charge up Kettle Hill at the battle of San Juan, Roosevelt's military exploits boosted his political career, leading him to become the youngest president of the U.S. at that time, following McKinley's assassination.
Step-by-step explanation:
The individual who played a highly visible role as a leader of the Rough Riders during the Spanish-American War before becoming president was Theodore Roosevelt. When the Spanish-American War broke out in 1898, Roosevelt resigned from his post as Assistant Secretary of the Navy to organize a volunteer cavalry known as the Rough Riders. He led a charge at the battle of San Juan, specifically up Kettle Hill, which cemented his reputation as a war hero. Despite the efforts of other groups, such as the African American Buffalo Soldiers, who played a crucial but often understated role in the battles, it was Roosevelt who emerged in the public eye as a hero, as documented in his memoir, 'The Rough Riders'.
Roosevelt's post-war popularity was significant, and following his heroic return, he was elected Governor of New York on the Republican ticket. This popularity would ultimately catapult him to the position of Vice President under William McKinley. With McKinley's assassination in 1901, Roosevelt assumed the presidency, becoming the youngest president in United States history at the time.