Back in the day, Justices didn't come from the corporate world because it didn't exist for lawyers, and many were "self taught".
Just weeks before Thomas Jefferson was to begin his presidency in 1801, incumbent John Adams appointed John Marshall as the young nation’s fourth chief justice. Generally considered to be the greatest jurist to fill that role, Marshall served under Jefferson, his political rival (and second cousin once removed), and four other presidents over the next three decades. Marshall studied law at William & Mary under the tutelage of George Wythe in 1780. Marshall’s tenure here was brief but potent in forming the character of the person who would lay the foundations of American constitutional law.
Largely self-educated, Marshall was born on September 24, 1755, in what is now Fauquier County, in the foothills of Virginia’s Blue Ridge Mountains. He was the oldest of 15 children of Thomas Marshall and Mary Randolph Keith. In youth he acquired a lifelong taste for English literature, poetry and history. Destined for the bar, Marshall set aside his law studies in 1775 to fight in the American War of Independence. As an officer in the Continental Army, he fought in the battles of Brandywine, Germantown, and Monmouth. He endured the sufferings at Valley Forge during the harsh winter of 1777-78. He was on leave from the army in 1780 when he attended Wythe’s lectures on law at the College. During his College sojourn, Marshall was elected to the Phi Beta Kappa Society. In nearby Yorktown he courted his future wife, Mary Willis (“Polly”) Ambler.