As a politician in a time when political corruption was widespread and patronage the norm, he was known for his integrity and independence – and for alienating more traditional politicians. A dynamic man, Roosevelt was also an accomplished historian, writer, and adventurer. He wrote biographies of politicians and a four-volume history, "Winning of the West" (1889-1896), in addition to books about nature and hunting.

His wealthy and socially prominent parents were divided over the Civil War; his father supporting the North and his mother supporting the South. Roosevelt was educated at home by private tutors because of his severe asthma; he also suffered from weak eyesight. His resolved to overcome his medical problems by physical exertion, leading to his lifelong interest in exercise and the outdoors. He attended Harvard where he studied to become a naturalist before developing an interest in politics and history. He studied law at Columbia University, but was bored and abandoned the legal profession in 1882 to begin his political career. He served in the New York State Assembly before being overcome by tragedy: on February 14, 1884 his wife died in childbirth; his mother died the same day. He left politics to spend two years on his cattle ranch in the Dakota Territory. (He experienced tragedy again when Quentin, his son by his second wife, was killed in World War One.)

Roosevelt became president, to a large extent, by accident. He was the Republican Governor of New York, but did not follow the party line. In order to remove him from that position, he was made the Vice-presidential candidate under William McKinley in the 1900 election. In 1901 McKinley was assassinated, and Roosevelt became president at age 42. In 1904 he was elected to a second term in a landslide. Rather than run for a 3rd term in 1908, he selected Taft as his successor – then, in the next election of 1912, opposed Taft unsuccessfully for the Republican nomination.