Sir Robert Gordon Menzies (20 December 1894 – 15 May 1978) was an Australian politician and lawyer who served as the 12th prime minister of Australia from 1939 to 1941 and from 1949 to 1966. He held office as the leader of the United Australia Party (UAP) in his first term, and subsequently as the inaugural leader of the Liberal Party of Australia in his second. He was the member of parliament (MP) for the Victorian division of Kooyong from 1934 to 1966. He is the longest-serving prime minister in Australian history.

Menzies studied law at the University of Melbourne and became one of Melbourne's leading lawyers. He was Deputy Premier of Victoria from 1932 to 1934, and then transferred to Federal Parliament, subsequently becoming Attorney-General of Australia and Minister for Industry in the government of Joseph Lyons. In April 1939, following Lyons's death, Menzies was elected leader of the United Australia Party (UAP) and sworn in as prime minister. He authorised Australia's entry into World War II in September 1939, and spent four months in England to participate in meetings of Churchill's war cabinet. On his return to Australia in August 1941, Menzies found that he had lost the support of his party and consequently resigned as prime minister. He subsequently helped to create the new Liberal Party, and was elected its inaugural leader in August 1945.

At the 1949 federal election, Menzies led the Liberal–Country coalition to victory and returned as prime minister. His appeal to the home and family, promoted via reassuring radio talks, matched the national zeitgeist as the economy grew and middle-class values prevailed; the Australian Labor Party's support had also been eroded by Cold War scares. After 1955, his government also received support from the Democratic Labor Party, a breakaway group from the Labor Party. Menzies won seven consecutive elections during his second period, eventually retiring as prime minister in January 1966. Despite the failures of his first administration, his government is remembered for its development of Australia's capital city of Canberra, its expanded post-war immigration scheme, emphasis on higher education, and national security policies, which saw Australia contribute troops to the Korean War, the Malayan Emergency, the Indonesia–Malaysia confrontation, and the Vietnam War.

Robert Gordon Menzies was born on 20 December 1894 at his parents' home in Jeparit, Victoria. He was the fourth of five children born to Kate (née Sampson) and James Menzies; he had two elder brothers, an elder sister Isabel, and a younger brother. Menzies was the first Australian prime minister to have two Australian-born parents: his father was born in Ballarat and his mother in Creswick. His grandparents on both sides had been drawn to Australia by the Victorian gold rush. His maternal grandparents were born in Penzance, Cornwall. His paternal grandfather, also named Robert Menzies, was born in Renfrewshire, Scotland, and arrived in Melbourne in 1854. The following year he married Elizabeth Band, the daughter of a cobbler from Fife. Menzies was proud of his Scottish heritage, and preferred his surname to be pronounced in the traditional Scottish manner (/ˈmɪŋɪs/ MING-iss) rather than as it is spelled (/ˈmɛnziz/ MEN-zeez). This gave rise to his nickname "Ming", which was later expanded to "Ming the Merciless" after the comic strip character. His middle name was given in honour of Charles George Gordon.

The Menzies family had moved to Jeparit, a small Wimmera township, in the year before Robert's birth. At the 1891 census, the settlement had a population of just 55 people. His elder siblings had been born in Ballarat, where his father was a locomotive painter at the Phoenix Foundry. Seeking a new start, he moved the family to Jeparit to take over the general store, which "survived rather than prospered". During Menzies's childhood, three of his close relatives were elected to parliament. His uncle Hugh was elected to the Victorian Legislative Assembly in 1902, followed by his father in 1911, while another uncle, Sydney Sampson, was elected to the federal Australian House of Representatives in 1906. Each of the three represented rural constituencies, and were defeated after a few terms. Menzies's maternal grandfather John Sampson was active in the trade union movement. He was the inaugural president of the Creswick Miners' Association, which he co-founded with future Australian Labor Party MP William Spence, and was later prominent in the Amalgamated Miners' Association.

Growing up, Menzies and his siblings "had the normal enjoyments and camaraderies of a small country town". He began his formal education in 1899 at the Jeparit State School, a single-teacher one-room school. When he was about eleven, he and his sister were sent to Ballarat to live with his paternal grandmother; his two older brothers were already living there. In 1906, Menzies began attending the Humffray Street State School in Bakery Hill. The following year, aged 13, he ranked first in the state-wide scholarship examinations. This feat financed the entirety of his secondary education, which had to be undertaken at private schools, as Victoria did not yet have a system of public secondary schools. In 1908 and 1909, Menzies attended Grenville College, a small private school in Ballarat Central. He and his family moved to Melbourne in 1910, where he enrolled in Wesley College. Menzies was "not very interested in and certainly incompetent at sport", but excelled academically. In his third and final year at Wesley he won a £40 exhibition for university study, one of 25 awarded by the state government.

In 1913, Menzies entered the Melbourne Law School. He won a variety of prizes, exhibitions, and scholarships during his time as a student, graduating as a Bachelor of Laws (LL.B.) in 1916 and a Master of Laws (LL.M.) in 1918. He did, however, fail Latin in his first year. One of his prize-winning essays, The Rule of Law During the War, was published as a brochure with an introduction by Harrison Moore, the law school dean. In 1916, Menzies was elected president of the Student Representatives' Council and editor of the Melbourne University Magazine. He wrote both prose and poetry for the magazine, and also contributed a song about "little Billy Hughes" to an end-of-year revue. Menzies was also president of the Students' Christian Union, a founding member of the Historical Society, and a prominent member of the Law Students' Society. He had "a reputation as an "unusually bright and articulate member of the undergraduate community", and was known as a skilful debater. However, he had also begun to develop the traits of pomposity and arrogance that would cause difficulties later in his career. His fellow law student and future parliamentary colleague Percy Joske noted Menzies as a student "did not suffer fools gladly [...] the trouble was that his opponents frequently were not fools and that he tended to say things that were not only cutting and unkind but that were unjustified".

During World War I, Menzies served his compulsory militia service in the Melbourne University Rifles (a part-time militia unit) from 1915 to 1919. This unit was not efficient and Menzies noted in his diary that training in even basic skills such as rifle shooting was sub-standard. He was commissioned a second lieutenant on 6 January 1915. Unlike many of his contemporaries, he did not volunteer for overseas service, something that would later be used against him by political opponents; in 1939 he described it as "a stream of mud through which I have waded at every campaign in which I have participated". Menzies never publicly addressed the reasons for his decision not to enlist, stating only that they were "compelling" and related to his "intimate personal and family affairs". His two older brothers did serve overseas. In a 1972 interview, his brother Frank Menzies recalled that a "family conference" had determined that Robert should not enlist. They believed that having two of the family's three adult sons serving overseas was a sufficiently patriotic contribution to the war effort, and that the family's interests would be served best by Robert continuing his academic career. Another reason for keeping one of the elder sons home was the health of their father, James, who was physically unwell and emotionally unstable at the time. It has been noted that, as a student, Menzies supported the introduction of compulsory overseas conscription, which if implemented would have made him one of the first to be conscripted. Promoted to lieutenant, he resigned his commission with effect from 16 February 1921.