Gilbert, Sir Humphrey (1537–83), soldier and explorer, was second son of Otho Gilbert (d. 1547) of Compton, near Dartmouth. Sir Walter Ralegh (qv) was his half-brother from the second marriage of his mother, Catherine, daughter of Sir Philip Champernoun. Gilbert was educated at Eton College and Oxford University, where he studied navigation, cosmography, and the art of war. In 1565 he projected an expedition to discover the north-west passage through America to the Pacific Ocean, but his plans later fell foul of the interests of the Muscovy company, and negotiations over the expedition broke down.
He first went to Ireland in September 1566 as captain in a military force dispatched to support the lord deputy, Sir Henry Sidney (qv), in his campaign against Shane O'Neill (qv). After landing at Carrickfergus, where his forces were involved in an engagement with O'Neill's men, Gilbert and his company proceeded by sea to Derry, where they joined Sidney who arrived there on 12 October, having marched through Ulster with his army. Gilbert then accompanied Sidney back to Drogheda, from where he was dispatched to England in November with letters to the queen and her chief minister, Sir William Cecil.
In May 1567 Gilbert returned to Ireland at the head of a newly raised company of soldiers. By then the death of Shane O'Neill seemed to offer an opportunity for the crown to advance its interests in Ulster, and either Gilbert or Sidney proposed the establishment of an English colony east of the River Bann. A consortium of Devon landowners and adventurers, of which Gilbert appears to have been a member, lobbied to receive a grant of land to found a settlement. By 1567 he was acting as the consortium's representative in Ireland, engaging as such in discussion with Sidney. In October 1567 he was appointed military governor of Ulster. However, the difficulties and costs associated with this colonial undertaking became more apparent and the proposed settlement was quietly shelved during 1568.
He continued his military service in Ireland until mid 1568, when he left for England, apparently suffering from ill health. After his return to Ireland in October 1568, he was elected MP in the 1569–71 Irish parliament and became involved in a scheme, again supported by his associates in Devon, to establish an English settlement in Munster. Initially, this scheme entailed merely founding a fishing colony at Baltimore, but in spring 1569 Gilbert and his colleagues requested a grant of all of Co. Cork and Co. Kerry, as well as any land confiscated by the crown from Irish rebels during the next ten years. They justified their demand for what was effectively a licence to conquer Munster with government assistance, by highlighting the strong and politically suspect trading links between Munster and Spain, and the resulting need to secure a strategically important region. The crown agreed to grant them most of Cork and Kerry, but rumours of this scheme contributed to the outbreak of a rebellion in Munster in summer 1569 led by James fitz Maurice Fitzgerald (qv).
While gathering his forces in Dublin, Sidney sent Gilbert as part of his vanguard into Kilkenny, where he showed great bravery in helping royal forces to beat off a rebel army besieging Kilkenny city in July. He then accompanied Sidney on his foray into Munster, where they obtained the surrender of some of the rebels, but the principals remained at large. On leaving Munster in September, Sidney appointed Gilbert to command the royal forces in the province. This appointment reflects Sidney's confidence in Gilbert's military prowess and the lord deputy's determination to proceed with the mooted colonisation scheme. However, even before the rebellion had started, the queen had become alarmed at the unrest caused by these schemes and refused Sidney's request to appoint Gilbert to the Irish privy council. For his part, Gilbert had not sought this promotion and requested leave to return to England. He commanded about 700 English soldiers, against whom were ranged several thousand Irish rebels, although he did have some Irish allies. More promisingly, Sidney vested in him unprecedented powers of martial law, which he exploited fully.
On 25 September he attacked a 1,500–2,000 strong rebel army with only 200 men near Kilmallock and put them to flight before undertaking a dangerous journey to Cork city and back again through rebel territory. In these encounters he displayed a reckless willingness to take the fight to the rebels. His apparent invulnerability led the Irish to regard him as an enchanter and as being more devil than man. His policy was that if a fort did not immediately yield to him, he would ignore all further offers of surrender and, after storming the fort, would put all in it to the sword. The massacre of the ward at Garrystown in October prompted the capitulation of over twenty-five castles. During October–November he campaigned in north Kerry and in the rebel fastness of Connellough woods. Short of men and provisions, he relied on terror to cow the local Irish into submission. His soldiers routinely killed all men, women and children in their path, as well as any livestock they encountered in order to deprive the rebels of sustenance. He refused to negotiate with the rebels or grant protections, demanding unconditional surrender. In order to highlight his mercilessness, he forced those who surrendered to walk to his tent through a corridor of severed heads. Those who did surrender and those who provisioned his men were not harmed. He also rode roughshod over the corporate privileges of the Munster towns, arguing that as a conquered nation, Ireland could only be ruled in an authoritarian and arbitrary fashion. His methods represented a bloody and radical departure from the norms of warfare in Ireland.
By these means, he deprived fitz Maurice of nearly all his allies and largely pacified Munster within the space of six weeks. The rebel leader fled to the Glen of Aherlow, the one part of Munster where Gilbert had to act with some restraint, as it lay within the lordship of Thomas Butler (qv), 10th earl of Ormond, who was a noted loyalist and a firm favourite of the queen. Gilbert complained of Ormond's reluctance to take action against fitz Maurice, but bowed to the earl's superior political weight. In late December he left Munster to meet Sidney at Drogheda, bringing with him a number of former rebels who had surrendered. Sidney showered him with praise, knighted him on 1 January 1570, and pardoned him and his men for the many atrocities they had committed. In late January Sidney yielded to his repeated entreaties and allowed him to depart for England. His absence was intended to be temporary, but he had no intention of returning and a new governor of Munster was appointed in late 1570. Although his governorship of Munster had lasted less than four months, both the memory of his brief reign of terror and the controversy generated by his conduct lasted for years. Fitz Maurice regrouped following Gilbert's departure and prosecuted a guerilla war for another three years with the covert aid of many of the lords who had surrendered. Gilbert's methods had broken the rebels’ grip on the province, but could not secure a lasting peace.
In 1572 he resurrected his earlier scheme for establishing a settlement at Baltimore, but again nothing came of it. Gilbert continued to be regarded as an authority on Irish matters, and wrote a memo to the English government in early 1574 arguing that the ongoing lawlessness of Ireland posed a grave strategic threat to England, particularly as the Irish rebels now sought aid from foreign powers such as Spain, and that the root of this lawlessness lay in the immoderate power enjoyed by the Gaelic Irish and Anglo-Norman feudal lords. He added that in order to subdue and successfully colonise remote Irish regions, royal officials could not allow themselves to be hindered by legal or humanitarian concerns.
On returning to England in 1570 he married a wealthy heiress, Anne, daughter of Sir Anthony Ager of Kent. They had six sons and one daughter. In 1571 Gilbert became an MP for Plymouth and made a name for himself by attacking Peter Wentworth, the chief protagonist for parliamentary power. He was rewarded by the queen with a patent to act as receiver general of fines for seven years. He then became involved in a mining and alchemical venture to transmute iron into copper, the failure of which involved him in considerable financial losses. In 1572, following the capture of Brill by Dutch rebels, Gilbert served alongside them in the Netherlands against Spain with a band of ‘volunteers’ for a few months. He went into retirement for the next five years, during which he produced a number of writings, including an educational report putting forward a scheme for a new type of university with an emphasis on practical rather than classical learning, and his Discourse of a discovery for a new passage to Cataia, written partly in support of his plans for a north-west passage to Asia. His interest in colonies now turned to America and on 11 June 1578 he obtained a charter for discovering, planting, and governing a colony. He set out in late 1578 but achieved little and was back in England by April 1579.
Following renewed unrest in Munster in summer 1579, he was sent with a fleet to patrol the coasts of Cork and Kerry and intercept any Spanish ships, continuing in this role until autumn 1580. In August 1579 he became involved in controversy when he robbed a Portuguese trading ship that was under the protection of Sir Owen O'Sullivan, lord of the Beare peninsula, leading to fighting between his men and O'Sullivan's. That October, he put his fleet into Cork city to give his men two weeks’ shore leave. Their behaviour, and his, provoked a flood of complaints. He attacked O'Sullivan in a tavern, and on leaving the city argued with and killed a local merchant. Although his fleet provided crucial support for the royal forces in Munster, his final period in Ireland proved expensive as he often had to pay his sailors’ wages out of his own pocket. Eventually, they deserted with two of his ships due to lack of pay, leaving him in financial straits.
On his return to England, Gilbert set about raising money for a second expedition to North America, and on 11 June 1583 he set out from Plymouth with four ships. They landed at St John's, Newfoundland, on 3 August, and Gilbert went ashore, taking possession of the harbour and the land within 200 leagues of it in the queen's name. He announced three laws for the government of the island, the earliest legislation for an English possession in America, before exploring the coast to the south. One of his ships sank in shallow water and he set sail for England at the insistence of his crews. His ship was overloaded with ordnance and nettings and at midnight on 9 September 1583 it sank during a storm. His death at sea left his wife in difficult circumstances and Sir Walter Ralegh petitioned for her to have a small income from the crown.