Field Marshal Garnet Joseph Wolseley, 1st Viscount Wolseley (4 June 1833 – 25 March 1913) was an Anglo-Irish officer in the British Army. He became one of the most influential British generals after a series of victories in Canada, West Africa and Egypt, followed by a central role in modernizing the British Army in promoting efficiency.

Wolseley is considered to be one of the most prominent and decorated war heroes of the British Empire during the era of New Imperialism. He served in Burma, the Crimean War, the Indian Mutiny, China, Canada and widely throughout Africa—including his Ashanti campaign (1873–1874) and the Nile Expedition against Mahdist Sudan in 1884–85. Wolseley served as Commander-in-Chief of the Forces from 1895 to 1900. His reputation for efficiency led to the late 19th century English phrase "everything's all Sir Garnet", meaning, "All is in order."

Lord Wolseley was born into a prominent Anglo-Irish family in Dublin, the eldest son of Major Garnet Joseph Wolseley of the King's Own Scottish Borderers (25th Foot) and Frances Anne Wolseley (née Smith). The Wolseleys were an ancient landed family in Wolseley, Staffordshire, whose roots can be traced back a thousand years. Wolseley was born at Golden Bridge House, the seat of his mother's family. His paternal grandfather was Rev. William Wolseley, Rector of Tullycorbet, and the third son of Sir Richard Wolseley, 1st Baronet, who sat in the Irish House of Commons for Carlow. The family seat was Mount Wolseley in County Carlow. He had four younger sisters and two younger brothers, Frederick Wolseley (1837–1899) and Sir George Wolseley (1839–1921).

Wolseley's father died in 1840 at age 62, leaving his widow and seven children to struggle on his Army pension. Unlike other boys in his class, Wolseley was not sent to England to attend Harrow or Eton, but was instead educated at a local school in Dublin. There, he enjoyed military history and the Latin classics, and excelled at mathematics, history, drawing, reading, and sports. However, the family circumstance forced Wolseley to leave school at age 14, when he found work in a surveyor's office, which helped him bring in a salary and continue studying mathematics and geography. The methodical nature of the job, with its need for attention to detail, would prove to serve him well in his future army career.

Wolseley first considered a career in the church, but his financial situation meant that he would have needed a wealthy patron to support such an endeavour. Instead he sought a commission in the Army. Unable to afford Sandhurst or buying a commission, Wolseley wrote to his fellow Dubliner, Field Marshal The 1st Duke of Wellington, for assistance. Wellington, then the Commander-in-Chief of the Forces, promised to assist him when he turned 16. However, Wellington apparently overlooked him and did not respond to another letter sent when he was 17. Wolseley unsuccessfully appealed to his secretary, Lord Fitzroy Somerset. The British Army was then recovering from significant casualties in the latest war in South Africa, and Wolseley wrote to Somerset, "I shall be prepared to start at the shortest notice, should your Lordship be pleased to appoint me to a regiment now at the seat of war." His mother then wrote to the Duke to appeal his case, and on 12 March 1852, the 18-year-old Wolseley was gazetted as an ensign in the 12th Foot, in recognition of his father's service.

Just a month after he joined the 12th Foot, Wolseley transferred to the 80th Foot on 13 April 1852, with whom he served in the Second Anglo-Burmese War. He was severely wounded when he was shot in the left thigh with a jingal bullet on 19 March 1853 in the attack on Donabyu, and was mentioned in despatches. Promoted to lieutenant on 16 May 1853 and invalided home, Wolseley transferred to the 84th Regiment of Foot on 27 January 1854, and then to the 90th Light Infantry, at that time stationed in Dublin, on 24 February 1854. He was promoted to captain on 29 December 1854.

Wolseley accompanied the regiment to the Crimea, and landed at Balaklava in December 1854. He was selected to be an assistant engineer, and attached to the Royal Engineers during the Siege of Sevastopol. Wolseley served throughout the siege, where he was wounded at "the Quarries" on 7 June 1855, and again in the trenches on 30 August 1855, losing an eye.

After the fall of Sevastopol, Wolseley was employed on the quartermaster-general's staff, assisting in the embarkation of the troops and supplies, and was one of the last British soldiers to leave the Crimea in July 1856. For his services he was twice mentioned in despatches, received the war medal with clasp, the 5th class of the French Légion d'honneur and the 5th class of the Turkish Order of the Medjidie.