Henry Docwra was born in 1564 at the old castle of Chamberhouse at Crookam in Berkshire. He was the son of Edmund Docwra and his wife, Dorothy, the daughter of John Golding of Belchamp St. Paul & Halsted, Essex, sister of the translator, Arthur Golding, and half-sister of Mary, Countess of Oxford. Due to financial difficulties, his father found himself obliged to sell Chamberhouse in 1585 and it may have been this circumstance which led Henry to enter the army at an early age.

Henry served as a soldier under Sir Richard Bingham in Ireland, where he attained the rank of captain, and was made constable of Dungarvan Castle from 20th September 1584. The campaign began on 1st March 1586, with the siege of the castle of Clonoan in Clare, then held by Mathgamhain O'Briain. After a siege of three weeks, the castle was taken and the garrison slain. The victorious army marched into Mayo and took the Hag's Castle, a medieval stronghold built upon an ancient crannog in Loch Mask. Bingham next laid siege to the castle of Annis, near Ballinrobe. The Joyces of Dubhthaigh-Shoigheach and the MacBonnels of Mayo rose in arms to support the fugitives from the Hag's Castle. Docwra's services seem to have commenced at this siege. On 12th July 1586, the force was encamped at Ballinrobe and, afterwards, made a series of expeditions till the tribes of Mayo were reduced. A force of Scottish highlanders having landed in alliance with the Burkes, it was necessary to march to Sligo to prevent their advance. Some of the O'Rourkes joined them on the Curlew mountains with McGuires from Oriel, and Art O'Neill, who afterwards went over to  Docwra, gave these clans some support. After an action in which the highlanders and their allies were victorious, Bingham's force was obliged to retire but, afterwards, defeated them at Clare in County Sligo. The Burkes, however, continued in arms and Bingham accomplished nothing more of importance.

Docwra left Ireland and commanded a regiment in the army of the Earl of Essex in Spain and the Netherlands. He was present at the Siege of Cadiz and was knighted in Spain. In 1599, his regiment, with that of Sir Charles Percy, was sent to Ireland to aid in suppressing the rebellion of Tyrone. Docwra took a prominent part in the war and was appointed, in 1600, to reduce the north. His army consisted of four thousand foot and two hundred horse, three guns and a regular field hospital of one hundred beds. He touched at Knockfergus (now Carrickfergus) on 28th April 1600 and remained there for eight days. On 7th May, he sailed for Lough Foyle, which he did not reach till the 14th. He landed at Culmore, where he found the remains of a castle abandoned by the English in 1567. These, he immediately converted, using earthworks, into a strong position. While these were being made, he marched inland to Elogh and garrisoned the, then empty, castle, the ruins of which remain on a small hill commanding the entrance from the south to Innishowen in Donegal. On 22nd May, he possessed himself of the hill now crowned by the cathedral of Derry. He must be regarded as the founder of the modern city of Derry, for he built streets as well as ramparts on the hilltop. O'Kane, with his tribe, lurked in the woods and cut off any stragglers. On 1st June, Docwra received the submission of Art O'Neill and, on 28th June, he fought his first serious engagement with the natives under O'Dogherty near Elogh. Docwra's force consisted of forty horse and five hundred foot. His lieutenant, Sir John Chamberlain, became unhorsed and, while the general endeavoured to rescue him, his own horse was shot from under him. The Irish captured some horses and retired from a battle in which what advantage there was rested with them. Docwra's courage won their respect and a local Gaelic historian says “he was an illustrious knight of wisdom and prudence, a pillar of battle and conflict”. A more serious battle was fought on 29th July with the O'Donnells and MacSwines, and the general himself was struck in the forehead by a dart, cast by Hugh the Black, son of Hugh the Red O'Donnell. He was confined to his room with his wound for three weeks and many companies in his army were reduced by disease and wounds to less than a third of their complement. On 16th September, he was nearly surprised by a night attack of O'Donnell and, next day, received much-needed food supply by sea.

Continued expeditions into the country employed Docwra for the whole winter and he penetrated to the extremity of Fanad. In April 1601, he reduced Sliocht Airt and, in July and August, made expeditions towards the River Ban, conquering O'Kane's country, and, in April 1602, obtained possession of the castle of Dungiven, commanding a great part of the mountain country of the present county of Londonderry. Besides warlike expeditions, he was engaged in endless negotiations with the natives. The war ended at the beginning of 1603, though it was only by great watchfulness that Docwra prevented a rising upon Queen Elizabeth's death. He remained as governor of Derry, with a garrison of about four hundred men, and immediately devoted himself to the improvement of the city. He received a grant, on 12th September 1603, to hold markets on Wednesdays and Saturdays, and for a fair. On 11th July 1604, he was appointed provost for life and received a pension of 20s a day for life. Two years later, he applied for the presidency of Ulster, but did not obtain it. So he sold his house, appointed a vice-governor and returned to England. Sir Henry had married Anne, daughter of Francis Vaughan of Sutton-upon-Derwent in Yorkshire, and they settled at Bradfield in Berkshire, with his widowed sister and her family; his young nephew, Edward Stafford, being lord of the manor there. Most of Sir Henry’s three sons and three daughters were born at Bradfield, but, in 1610, he also purchased the minor manor of Idstone at Ashbury, not far away. He published, in 1614, 'A Narration of the Services done by the Army employed to Lough Foyle under the leading of me, Sir Henry Docwra, knight.' He had previously written 'A Relation of Service done in Ireland,' being an account of Bingham's campaign. He was appointed treasurer of war in Ireland in 1616, sold-up his English estates and returned to live there, being raised to the peerage as Baron  Docwra of Culmore on 15th May 1621.

On 15th July 1624, Sir Henry was appointed keeper of the peace in Leinster and Ulster, and, on 13th May 1627, joint keeper of the great seal of Ireland. He was one of the fifteen peers appointed, on 4th June 1628, to try Lord Dunboyne, and he was the only one who voted for a conviction. He died in Dublin, on 18th April 1631, and was buried in the cathedral of Christ Church. His eldest surviving son, Theodore, succeeded him in the title, but died without issue, when the barony became extinct.

Docwra resembled the soldiers who, in later times, increased the British dominion in India. He was a skilful commander, whose personal intrepidity won the respect of his own men and of the enemy, and he followed a consistent plan of wearing out the hostile tribes by constant activity, by preventing their junction, and defeating them in detail. At the same time he took advantage of every quarrel in the native families, and was ready to support as the rightful one whichever claimant submitted to England, and without scruple as to the real merits of the case. Except in this respect, his conduct was invariably honourable and he showed more public spirit and less anxiety for his own emolument than was common in his age and field of service.