William Henry Quilliam (10 April 1856[1][2][3] – 23 April 1932), who changed his name to Abdullah Quilliam and later Henri Marcel Leon or Haroun Mustapha Leon, was a 19th-century British convert from Christianity to Islam, noted for founding England's first mosque and Islamic centre, and Britain's oldest Muslim organization, the Association of British Muslims.

Quilliam purchased numbers 8, 11 and 12 Brougham Terrace, Liverpool, following his conversion, thanks to a donation from Nasrullah Khan, Crown Prince of the Emirate of Afghanistan.

In addition, the Institute hosted educational classes covering a wide range of subjects, and included a museum and science laboratory.

[4] Quilliam also published The Crescent, a weekly account of Muslims in Britain, and Islamic World, a monthly publication with a worldwide audience.

Abdul Hamid II, the 26th Ottoman Caliph, granted Quilliam the title of Shaykh al-Islām for the British Isles.

[4] He had contact with English-speaking West African Muslims and toured the region's coastal cities on his way to Lagos to attend the consecration of the Shitta Bey Mosque in 1894.

The prominent Anglo-Muslims Abdullah Yusuf Ali, Muhammad Marmaduke Pickthall (who each translated the Qur'an), and Lord Headley were later buried near him.

[13] During the war in Sudan, Quilliam published a pamphlet stating that any British Muslim that decided to aid in some manner the expedition was acting in "contrary to the Shariat".