Samuel Liddell (or Liddel) MacGregor Mathers (8 January 1854 – 5 or 20 November 1918), born Samuel Liddell Mathers, was a British occultist and member of the S.R.I.A. He is primarily known as one of the three founders of the Hermetic Order of the Golden Dawn, a ceremonial magic order of which offshoots still exist. He became so synonymous with the order that Golden Dawn scholar Israel Regardie observed in retrospect that "the Golden Dawn was MacGregor Mathers."

Mathers was born on 8 January 1854 in Hackney, London, England. His father, William M. Mathers, a commercial clerk, died while he was still a boy. He lived with his widowed mother (whose maiden name was Collins) in Bournemouth, until her death in 1885. Mathers attended Bedford Grammar School and subsequently worked as a clerk, before moving to King's Cross in London, following the death of his mother.

Mathers met Mina Bergson, sister of philosopher Henri Bergson, in the British Museum Reading Room where he spent much of his time. The two had an immediate rapport and were married on 16 June 1890, despite the opposition of Mina's family. The couple lived in Forest Hill until they had to move to central London due to poverty. There, they were financially supported by Annie Horniman, a wealthy tea heiress who helped Mathers get a job at her family museum and eventually joined the Order of the Golden Dawn. Horniman continued supporting the couple until 1896.

Mathers was fascinated by Celtic Symbology and his purported Highland Scottish ancestry from an early age. According to his wife, he was related to Ian MacGregor of Glenstrae, an ardent Jacobite who went to France after the 1745 uprising and fought at the Battle of Pondicherry, under Thomas Arthur, comte de Lally. Louis XV made him Comte de Glenstrae for his services. Mathers adopted the "MacGregor" prefix to honour his ancestor.

As a young man, Mathers became acquainted with Kenneth R. H. Mackenzie, a noted Freemason and occultist, who was friends with Edward Bulwer-Lytton. It was through Bulwer-Lytton and Anna Kingsford that Mathers got introduced to Helena P. Blavatsky in 1887. Blavatsky invited Mathers to collaborate with her in the formation of what later became known as the Theosophical Society, but, notwithstanding his admiration for Blavatsky, Mathers declined her invitation as there were some important differences between her philosophy and his.

According to the records of the Hermetic Order of the Golden Dawn, the organization's foundational documents, called the Cipher Manuscripts, were passed from Mackenzie to the Rev. A. F. A. Woodford. In turn, Woodford passed them on to Freemason William Wynn Westcott in 1886, who managed to decode them a year later, and upon doing so, called on Mathers for a second opinion. Mathers is credited with the design of the curriculum and rituals of the Order, and together with Woodford and Westcott, he is considered as one of the three founders.

Mathers was a practicing vegetarian, or (according to some accounts) vegan, an outspoken anti-vivisectionist, and a non-smoker. He was also a supporter of women's rights and he had little interest in money. It is known that his main interests were magic, military tactics and warfare, his first book being a translation of a French military manual, Practical Instruction in Infantry Campaigning Exercise (1884). He was also a keen student of boxing and fencing.

Mathers became increasingly eccentric in his later years as was noted by W. B. Yeats.