He was admitted to the bar the same year and commenced practice in Baton Rouge, Louisiana.

Robertson was elected a member of the State house of representatives in 1879.

He was reelected to the Fifty-first and to the eight succeeding Congresses, serving from December 5, 1887, to March 3, 1907.

[1] He served as chairman of the Committee on Levees and Improvements of the Mississippi River (Fifty-second Congress).

He was an unsuccessful candidate for renomination in 1906, and resumed the practice of law in Baton Rouge.