He treated settler Donald McIntyre of Kayuga after McIntyre was attacked by assigned servant Edward Gills in February 1834. 
He was registered as a Licensed Medical Practitioner for the district of Gammon Plains in 1839 and was witness at the trial of John Martin who was convicted of murder in 1840. 
He owned the premises of the Golden Fleece Inn, innkeeper John Perfrement, at Merriwa which he was advertising to sell in 1853.
He married Matilda Leard at West Gosford in 1847.
In March 1847 he moved to Maitland where he intended establishing a practice as Surgeon and Accoucheur. He took up rooms at Mr. Pinhey's Medical establishment in High Street and Hunter Streets, West Maitland where he could be consulted daily. 
Following is one of the cases he dealt with in 1847:
'On Friday evening last, asMr. William Harperof Harper's Hill, was returning homewards, about eight o'clock a cow ran at him as he was passing through a lane and gored him in the chest, the horn passing through into the lungs. Dr. McCartney was sent for immediately the unfortunate accident became known to the family, and reached there about three in the morning; he found Mr. Harper in a very dangerous state, and bled him copiously. Mr. Harper continued in so precarious a state for several days that Dr. McCartney found it necessary to be constantly by his bedside, the inflammation being only kept down by copious bleeding, applying leeches, etc. yesterday we were glad to hear Mr. Harper was considered out of danger, though the almost constable presence of a medical man was still requisite.'
Dr. McCartney's practise was later situated in Gorrick's buildings near the Fitzroy Hotel and he offered his services as a Surgeon and Accoucheur here in November 1847.
He was house surgeon for Maitland Hospital and in December 1847 was calling for tenders for the building of the Hospital. His salary for services to Maitland Hospital in January 1848 was £ 7/10/0-.
The Use of Ether at Maitland Hospital
In July 1848 he assisted Dr. David Sloan in an operation in Maitland on a four year old boy from Merriwa. .........
'On Sunday morning last a dangerous and difficult operation was performed by Dr. Sloan with the aid of chloroform, on a little boy named Wickes. It appears that for some months past the little boy, who resided near Merriwa, has suffered from a malignant tumour in the right nostril, filling up all the space between the eye and the nostril. The tumour was of a dangerous character, and although medical assistance was called in, it continued to get worse. Mrs. Wickes at length resolved to bring her son to Maitland, and placed him under the care of Drs. Sloan and McCartney. After an examination of the tumour it was determined to dissect it out, and on Sunday morning accordingly Drs. S. and McC. subjected the child to the influence of chloroform, which immediately made him insensible; Dr. Sloan at once laid open the tumour, and the operation was performed with complete success, the whole of the tumour being effectually removed; the operation proved a very long one, from the excessive bleeding, and the dangerous extent of the diseased part; the most painful portion of the operation was accomplished while the child was insensible, and on his reviving he was a second time subjected to the action of chloroform, and became again insensible. The little boy, who is a very strong and active child, has gone on very well since the operation. It is remarkable that the chloroform used on this occasion was the remainder of that obtained from Sydney for the purpose of operating on the poor man Ryan, in the hospital; it will be remembered that the chloroform rendered Ryan violently excited and nervous, without making him insensible; while on the little boy Wickes, the effects were all that could be desired, producing immediate insensibility. These facts prove that this powerful agent will produce effects governed by the constitution and possibly the habits of life, of the individual subjected to its influence; although, judging from the cases reported in the English and colonial papers, immediate insensibility will be produced by it in the great majority of cases'
There was no ambulance system in the 1840's. When a young orphan immigrant girl was badly burned, the skin being entirely burned from both legs from the feet to hips, both arms from hands to shoulders and her neck and body, Dr. McCartney devised a temporary method of conveying her to the Maitland Hospital so that she might suffer as little as possible. He stretched a quilt across a spring cart and placed a mattress on it and the girl was then placed on the mattress and conveyed to the Hospital.
In 1849 Dr. McCartney was recommending 'Burnett's Patent Disinfecting Fluid'. It had become possible to purchase this disinfectant from surplus stock of ships arriving from England and Dr. McCartney had used it with good effect at the old Maitland Hospital, 'removing entirely the strong and peculiar smell which is found in hospital wards, particularly when overcrowded'. It was found to be most useful in driving away bugs from wooden bedsteads! He had also used it in his own house and sick rooms with the same beneficial effects and considered the 'inodorous' fluid one of the most valuable discoveries of medical science.
Michael McCartney died on 22nd July 1860. He was buried in the Campbell's Hill Burial Ground