Again leaving Kennedy, he set out to make a very extended excursion.  Traversing the country from the head of the Maranoa, he discovered the Warrego River.  Keeping north, over the watershed, for a time he fondly imagined that he had reached northward-flowing waters; but the direction of the rivers that he found, the Claude and the Nogoa, soon convinced him of his error, and that he was on rivers of the east coast.  Even when he had reached the Belyando, a river which he named and followed down for a short distance, he still deluded himself that he had reached inland waters.  Intensely mortified at finding that he was on a tributary of the Burdekin, and approaching the ground already trodden by Leichhardt, he returned to the head of the Nogoa, once more subdivided his party, and formed a stationary camp to await his return from a westward trip.

This time, however, he was blessed with the most splendid success.  He found the Barcoo, a river that seemed to him to promise all he sought for.  The direction of its upper course easily led him to believe that it was an affluent of the Gulf of Carpentaria, and after tracing it for some distance he returned to camp.  The newly-discovered river he named the Victoria, thinking it would prove to be the same as that found by Captain Stokes on his survey expedition.  It was on the Barcoo, or Victoria, that Mitchell first noticed the now famous grass that bears his name.  On their return journey, they followed down the Maranoa, and at the old camp at St. George’s Bridge, they were told by the natives that white men had visited the place during their long absence.  It was a singular and welcome feature of Mitchell’s discoveries that they had always proved to be adjacent to civilisation, and to be suitable for immediate occupation.

The discovery of the Barcoo was the last feather in the cap of the Surveyor-General.  He was doomed to learn soon that it was not the river of his dreams, but only the head waters of that central stream discovered by Sturt, Cooper’s Creek; but meanwhile the delusion must have been very gratifying.

In 1851 Mitchell was sent out to report on the Bathurst goldfields, and on a subsequent visit to England he took with him the first specimen of gold and the first diamond found in Australia.  He was for a short time one of the members for the Port Phillip electorate, but resigned, as he found faithful discharge of the duties to be incompatible with his office.  He patented the boomerang screw propeller, and was the author of many educational and other works, including a translation of the Lusiad of Camoens.  Although a strict martinet in his official duties, and subject to a choleric temper, he was strenuous in his devotion to the advancement of Australia, among whose makers he must always occupy a proud position.  He died on the 5th of October, 1855, at Carthona, his private residence at Darling Point, Sydney, New South Wales.  His wife was a daughter of Colonel Blount.