After graduating, he was admitted to the bar in 1843, settled in Charlottesville to practice law, and won election as Commonwealth's Attorney for Albemarle County, Virginia in 1852.

His Charlottesville home, the Judge William J. Robertson House, was listed on the National Register of Historic Places in 1999.

[2] In 1860, on the death of Peter V. Daniel, some Virginians lobbied for President James Buchanan to select Robertson for the United States Supreme Court.

[5] Returning to private practice, he was attorney in many important cases involving the interests of Virginia and her citizens following the war.

[8] His ideas "fell like a thunderbolt on some of his hearers," reported the editor of the Virginia Law Journal, who concluded, "I doubt they will recover their serenity in a year.