Robert Armistead McLean was born on January 6, 1851, in Stockton, California. His parents, both natives of Virginia, were Dr. Samuel Merryweather McLean and Ellen Grey Jeter McLean. The elder Dr. McLean had built the first hospital and the second frame house in Stockton. Robert McLean spent most of the Civil War with an uncle in Bethlehem, Pennsylvania. In 1866 he returned to the family home, then located at Copperopolis, California.

At the age of eighteen he became an apprentice in the study of medicine under his father. Samuel McLean was friendly with Hugh Toland (founder of the Toland Medical College which, in 1872, became the medical department of the University of California), and arranged for Toland to become the preceptor of his son, Robert. Robert McLean was graduated from the University of California's medical department in 1874; he was the first native Californian to receive the degree of Doctor of Medicine. Dr. McLean became a member of the faculty of the medical school as lecturer in anatomy and as secretary to the dean, Dr. R. Beverly Cole. During this period, he also continued his association with Dr. Toland, whom he assisted in his private practice and in his duties as professor of surgery at the county hospital. Soon he was promoted to the position of professor of anatomy and, when Dr. Toland died in 1880, Dr. McLean succeeded to the chair of clinical and operative surgery. He held this professorship until his retirement in 1899. From 1881 until 1899 he was also dean of the college.

Dr. Toland, in his will, had bequeathed "to the most competent man to succeed me, my office, furniture, instruments, library, and practice." The office, located at 603 Merchant Street, at the corner of Montgomery Street, was where Dr. McLean commanded the largest surgical practice in California until 1897 when, at the age of 43 and at the height of his activity, he was rendered hemiplegic by a stroke. Following a period of struggling to recuperate, he regained his ability to speak and eventually was able to walk with the aid of a cane, but was troubled by unilateral ataxia for the next twenty years of his life.

The year 1897 brought another blow to Dr. McLean's life when his wife Alice died. The couple's sixteen-year union had resulted in three sons. Though none of the sons followed their father into the medical profession, it is interesting to note that Dr. McLean's sister Bessie (who married one of Dr. McLean's honor students, Dr. C. W. Evans) was the mother of Dr. Herbert McLean Evans, who had a distinguished scientific career as an endocrinologist at the University of California.

While the stroke effectively ended Dr. McLean's booming surgical practice, from 1906 to 1915 he occupied an office at the corner of Ellis and Powell streets with an old colleague, Dr. George H. Powers. One writer, Dr. Robert T. Legge, has noted that Dr. McLean "enjoyed quite a little practice among old friends and charity patients ... the number of types of minor office surgery he performed was surprising. Dr. McLean was apparently more disappointed by the end of his teaching career, which ground to a halt when, following the stroke, he was not invited to resume his teaching activities at any level. So wounded was he by this rebuff, it has been said, that for the remainder of his life he never set foot inside the buildings of the medical school again.

Dr. McLean published a handful of papers during his career. Two of these were on cancer, one on tuberculosis; his 1880 speech before the graduating class was published in Western lancet, and his appreciation of Hugh Toland was published in Cyclopedia of American Surgeons in 1912. His interest in young men's leading clean and industrious lives caused him to publish, in 1908, a small monograph titled Letters of Advice to a Young Man. In 1914 Dr. McLean brought together several of his college associates to form the Toland Medical Club, a social organization "modelled after the immortal 'Pickwick Club' of Dickens ... in order to discourage all scientific discussion and to foster good fellowship and a spirit of fraternity ..."

In the last days of his retirement he resided at Cloyne Court, in Berkeley. He was at this location when the Spanish influenza epidemic of 1918-1919 occurred. On December 4, 1918, Robert A. McLean died at Berkeley's Alta Bates Hospital as a result of influenza and pneumonia.