His appointment as Surveyor-General of New South Wales, as successor to John Oxley, took place in 1827, when he at once assumed office, and started energetically to lay out and construct roads, then the urgent need of the new colony.
His strong personality, and the energy and thoroughness he displayed in all his undertakings, combined with his many gifts as draughtsman, surveyor and organizer, proved to be of peculiar service to the colony at that period of its existence.

There was a vast unknown country surrounding the settled parts, awaiting both discovery and development, and Mitchell's inclinations and talents being strongly directed towards geographical discovery, the office of Surveyor-General that he held for so long was the most appropriate and advantageous appointment that could have been given him in the interests of the colony.
At the same time, Major Mitchell had faults which have always detracted from the estimation in which he would otherwise be held for his undoubted capabilities.

His domineering temper led him into acts of injustice, and often made it impossible for him to allow the judgments of others to influence his opinions.

In his view, no other explorer but himself ever achieved anything worthy of commendation or propounded any credible theory regarding the interior of Australia.

He always referred slightingly to Sturt, Cunningham, and Leichhardt, and his perversity on the subject of the junction of the Darling and the Murray drew even from the gentle Sturt a richly-deserved and unanswerable retort.