All Leslies are descended from the same Scottish Clan and Great Family, one of the very oldest. Tradition says it began in 1069 A.D. when the Saxon claimant to the throne of England was defeated by William the Conqueror and fled to Scotland. Edward the Atheling, as he was called, arrived there with his mother, a Hungarian Princess, and his two sisters. They were cordially received by the king, Malcolm III Canmore. A year later the king married Margaret, one of the sisters, who became a great queen of Scotland and a saint of the Christian Church. Her Chamberlain, Bartholomew, a Hungarian nobleman, had much impressed King Malcolm who made him Commander of Edinburgh Castle. Bartholomew married Beatrix, the King's sister in 1070 and thus the family and Clan of Leslie was founded. Bartholomew was granted lands at Leslie in the district of the Garioch, near Aberdeen, originally known as the lands of Lesslyn. It is from Lesslyn that the Clan eventually took its name. The present Leslie Castle, built near site of the first primitive stronghold, recently was completely restored by architect David C. Leslie.

Bartholomew's son Malcolm was knighted by King David and had the Leslie lands confirmed to him by royal charter, the oldest personal land charter in Aberdeenshire. Malcolm's grandson, Sir Norman, was the first to have the family name recorded in a charter. He was one of the magnates of Scotland who sat in the parliament called by King Robert the Bruce in 1314. Sir Norman's son Andrew, the 6th Lord Leslie, was one of the great barons of Scotland who in 1320 signed the Declaration of Arbroath, a call to freedom. With Andrew's sons begins the spread of the family through several branches. The original line died out with Andrew's grandson David, but son Walter became the Earl of Ross, son John the progenitor of the Earls of Rothes, and son George the first Baron of Balquhain.

The third line, Leslie-Rothes, became the senior line with George who was created the first Earl of Rothes around 1457. George, 4th Earl of Rothes, had a significant diplomatic role in the reign of James V; Andrew, 5th Earl, was of importance to Mary, Queen of Scots and King James VI; John, 6th Earl, was a leader of the Scottish opposition to King Charles I and a champion of the National Covenant of 1638; John, 7th Earl, became General of the Army of Scotland during the reign of Charles II, Lord High Chancellor, and was created a duke. Subsequent Earls had high military and civil positions and became hereditary Representative Peers of Scotland in parliament until 1965 and the 20th Earl of Rothes, when this system was abolished. Ian, the present and 21st Earl of Rothes, Chief of Clan Leslie, as a hereditary peer was entitled to attend the House of Lords until this hereditary right was abolished in 1999.

The fourth line, the Leslies of Balquhain, became the largest and most prolific. Except for those very few directly related to the Rothes, just about all Leslies today are descended from the Balquhain, in particular via the Third Baron, Andrew. The Balquhain Leslies for centuries played important, often colorful part in Scottish and other history. From Andrew were descended a host of families, among them the Earls of Leven, Lords Newark, the Leslies of Pitcaple, Kininvie, Iden and Cults; in Ireland the Leslies of Kincraigie, Glaslough and Tarbert; the Lords Leslie in Russia, Counts Leslie in France and in the Holy Roman Empire. From these and other branches, many Leslies descended, who spread from Scotland all over the world. Many came to the United States and Canada, starting in the 18th century, mainly in the 19th. Leslies had transferred, in the 17th century, from Scotland to Ireland and a majority of those who came to the United States were descended from those who had gone to Ireland first.

Individual Leslies over the centuries gained fame as soldiers, participating in the military of Scotland, the United Kingdom, Sweden, Denmark, Germany, Poland, Russia, Austria, and the United States. The Leslies delivered many distinguished men of the cloth, such as John Leslie, Bishop of Ross, Counselor to Mary Queen of the Scots, and a later John Leslie, the famous royalist "Fighting Bishop" of Clogher in Ireland. Leslies gained notable success in the worlds of intellect and the arts, among them Sir Harald, jurist, created Lord Birsay; Sir John, physicist; Charles Robert, painter; Henry David, composer; Edith, educator; Mary Isobel, Lionel, and Eliza, writers; Peter, geologist; Thomas Edward, economist; Donald, sinologist; Ada, actress; Sir Sean, historian. There are over 1200 works by Leslies of various professions in the U.S. Library of Congress in Washington D.C.

The Clan Leslie Society was founded in 1978. Ian, the 21st Earl of Rothes, appointed the Honorable Alexander Leslie Klieforth, an American Diplomat, as his Commissioner in the United States of America with authority to setup and organize a Clan Society. The CLS was then founded, members recruited and its constitution and by-laws which govern the Society approved. They were ratified in 1980 in Charleston, SC, USA, which also elected the Society's first Chieftain and slate of officers. CLS International Gatherings are held biennially in various locations.