[1] Napton's father was a moderately successful tailor, enough so that as the oldest son William received private tutoring before enrollment in Princeton and Lawrenceville (New Jersey) preparatory academies.

[1] Napton then entered Princeton College in 1824 at the age of sixteen as a Junior, his previous studies and knack for Latin and Greek languages allowing him to bypass his freshman and sophomore years.

[1][2] Following his graduation in 1826 Napton moved to Albemarle County, Virginia, where he was employed as a tutor for the children of General William F. Gordon for a period of two years.

[2] Settling briefly in Columbia, William Napton soon moved to the Howard County town of Fayette, then a hotbed of Missouri politics.

This group, collectively known as the "Central Clique—Jacksonian Democrats and acolytes of Missouri Senator Thomas Hart Benton—dominated the state's politics in pre-Civil War times.

[3] With the assistance of his politically powerful friends in the Central Clique, William Napton was elected secretary of the Missouri State Senate in September, 1834, a position he held concurrently with his duties at the newspaper.

[1] In December 1836 Missouri Governor Lilburn Boggs, another Clique member, appointed Napton the state's fourth Attorney General, replacing Robert W. Wells, who had resigned to take a Federal judgeship.

[2][6] It was a history-making time to be in Missouri government, with major issues like the Honey War—a border dispute with Iowa, the 1838 Mormon War and Governor Boggs' Executive Order 44 in the forefront.

He and Justice James Harvey Birch, held the view that the Missouri Compromise was unconstitutional and that the U.S. Congress lacked the legal power to legislate slavery in territorial areas.

[2] Following his 1851 election defeat, and with a large family to support, William Napton became a circuit lawyer, traveling throughout the state but mainly the counties along the Missouri River.

Arrested by Federal troops in St. Louis, he was not allowed to begin his law practice until he swore the loyalty oath that had cost Napton his Supreme Court position.

[6] Initially appointed to fill the term of Justice Ephraim B. Ewing, who died suddenly, Napton was elected to the court in his own right the following year by over fifty thousand votes.