William Buchanan was a son of Captain Andrew Buchanan of Mersheen, Ireland.  He arrived in New South Wales on the Mangles in 1822. Other free passengers on the Mangles included Mrs. Wall, Mrs. Cogill, Mr and Mrs Timothy Nowlan and child; Mr and Mrs Simpson 2 children and 7 servants.
His brother was Charles Henry Buchanan of the 69th regiment who arrived on the Statesman with his wife and family in January 1837 and settled near Invermein.
William Buchanan applied for a town allotment in Sydney in June 1824, however did not take up that which was offered......
In consequence of the plan of Sydney being shown to me by Mr. Oxley and an allotment in Cockle Bay pointed out as vacant, I went there and saw what I supposed to be that allotment; there was a dyke on the eastern side; on the land inside that dyke there was a lime kiln; I was desirous of obtaining a piece of cleared land on which to build a cottage to reside in. I did not take it up but preferred to wait till I obtained another more fit for the purpose which I was promised. 
William Buchanan received a grant of 1000 acres in June 1824 with an annual Quit rent of £7 10s.(9) He selected this land and named it Mersheen, however he was a civil engineer at Norfolk Island in the following two years and did not have the financial means to reside on his grant. 
Norfolk Island Pirates
He was passenger on the brig Wellington in February 1827 when it was piratically taken by convicts on the voyage to Norfolk Island. The pirates made for New Zealand where they were captured by Captain Duke in the Sisters and conveyed back to Sydney. It is one of the most remarkable stories in colonial history and excited enormous public interest, in part because of the notorious reputation of Norfolk Island penal settlement. Five of the pirates were later executed.
William Buchanan gave evidence at the trial: Mr. William Buchanan stated, that he was a passenger on board the brig Wellington, proceeding to Norfolk Island ; on the 21st December about half past 11 o'clock in the morning he heard a noise and turning round he saw some of the prisoners engaged with the crew; he opposed himself to them, and was knocked down by a blow of a musket from one of the prisoners, and lay senseless for some time; on recovering he found himself confined in his cabin. Walton and Douglas, and about a dozen more prisoners whom he could not identify, came down into the cabin and demanded the charges and nautical instruments; cannot tell the name of the man by whom he was knocked down but believes he is a amongst the prisoners at the bar; the charts were given up to Walton and the ship's place pointed out by the master; the prisoners took possession of all the arms; Edwards took upon himself the direction of the guard; O'Neil acted as serjeant; Browne as a sailor, and Daley occasionally carried a musket or cutlass; does not remember any of the others who carried arms; did not consider himself confined, as he went on deck when he pleased, though there was a sentinel stationed below; the military were put in irons. Cross examined. Walton, Douglas and Clary took the first part in the transaction; they frequently said they regained that, it was their intention to give up the brig and cargo to the master; witness understood that Walton direction the ship's course; cannot say exactly but always thought that Walton took the rate of going which was afterwards calculated by the master. 
After finishing at Norfolk Island William Buchanan was employed as Assistant Engineer at Liverpool and in 1831 he was Superintendent of Convicts and Public Works and Deputy Postmaster at Newcastle. The cottage he probably occupied while at Newcastle can be seen situated close to the harbour on the 1830 Map of Newcastle by John Armstrong.
In 1832 he was appointed Clerk of Works in Sydney . In 1837 he was appointed Clerk of Works at Norfolk Island 
He may not ever have resided on his grant at the Hunter River, however convicts were assigned to him in the Hunter Valley so there were probably huts or cottages and stockyards constructed. He owned a herd of 387 cattle by 1831 which he was attempting to sell to the Australian Agricultural Company in that year.
Employed as Surveyor
In January 1838 it was announced that William Buchanan, for many years Superintendent of Works at Newcastle and afterwards clerk of Works in the Engineer's Office had been appointed Surveyor under the Sydney Building Act . He continued in this position in Sydney until 1846 when the position was abolished. He unsuccessfully petitioned the Council for compensation and finally took his case to the Legislative Council.
He purchased a town allotment at Muswellbrook in November 1838.
Convicts assigned to William Buchanan included -
John Eagan per Earl St. Vincent assigned servant in Sydney 17 October 1823
Daniel Ferguson per Three Bees assigned servant in Sydney 20 February 1824
Joseph Lees per Neptune assigned servant in Durham Co. in 1828
James Payne per Emma Eugenia. Absconded from service at Muswellbrook in 1840
Christopher Grist per Sophia. Assigned to William Buchanan at Hunter River in 1829
Michael Gaul per Sophia. Assigned to William Buchanan at Hunter River in 1829
Hamilton C. Sempill
In 1839 the Sydney Gazette advertised that 1200 acres of land promised to William Buchanan had been granted to Hamilton Collins Sempill.
 Australian Dictionary of Biography
 Sydney Gazette 21 February 1827
 Sydney Gazette 29 March 1832
 Sydney Gazette 27 October 1832
 Sydney Gazette 6 July 1837
 Sydney Herald 25 January 1838
 In the Service of the Company: letters of Sir Edward Parry, Commissioner to the Australian Agricultural company: volume 1, December 1829 - June 1832 Letter 459
 Wood, W. Allan, Dawn in the valley : the story of settlement in the Hunter River Valley to 1833, Wentworth Books, 1972. p. 98
 Index to map of the country bordering upon the River Hunter... by Henry Dangar (London : Joseph Cross, 1828). p20