Final answer:
John Quincy Adams wrote a biography of his father John Adams, making him the first president to write a biography of another president.
Step-by-step explanation:
The first president to have written a biography of another president was John Quincy Adams. He authored a biography of his father, John Adams, who was the second president of the United States. John Quincy Adams himself was the sixth president and has the unique distinction of being the only former president to serve in Congress after his presidency, where he was a strong advocate against slavery and for the freedom of speech.