The eldest son of Garcia Sarmiento de Sotomayor, corregidor of Granada, and governor of the Canary Islands, by his marriage with Juana de Acuña, an heiress, Diego Sarmiento was born in Gondomar, Galicia. He inherited large estates both in Galicia and in Old Castile. In 1583 he was appointed by King Philip II of Spain to the military command of the Portuguese frontier and sea coast of Galicia. He is said to have taken an active part during the Anglo-Spanish War in repelling an English raid on the coast in 1585, and in defending La Coruña against an unsuccessful English attack by Sir Francis Drake and the "English Armada" in 1589. In 1593 he was named corregidor of Toro. In 1603 he was sent from court to Vigo to superintend the distribution of the treasure brought from America by two galleons, which were driven to take refuge there, and on his return was named a member of the board of finance. In 1609 he was again employed on the coast of Galicia, this time to repel a naval attack made by the Dutch. Although he held military commands, and administrative posts, his residence was at Valladolid, where he owned the Casa del Sol and was already collecting his fine library. He was known as a courtier, and apparently as a friend of the favourite, the Duke of Lerma. In 1612 he was chosen as ambassador in England, but did not leave to take up his appointment till May 1613.
Gondomar's friends at court, the bien intencionados of his dispatches centered upon the Howards, Henry Howard, 1st Earl of Northampton (died 1614), Thomas Howard, 1st Earl of Suffolk, Lord High Treasurer, whose daughter was married to James's favourite, Charles Howard, 1st Earl of Nottingham, Lord High Admiral, Thomas Howard, 21st Earl of Arundel, and their protegés. The "Howard faction" preferred a marriage alliance with Spain over one with France, the traditional ally of Scotland, and they preferred to keep out of open warfare with Spain promoted by the more zealous Puritans. Most of the Howards were Catholics, encouraging them towards a spirit of toleration at home. Like many at the English court, they were receiving pensions from Spain, without much effect on their opinions and actions, and Gondomar seldom had the money to follow through. Among the pensioners, in an embarrassing list that surfaced in 1613, at Gondomar's first arrival was the King of England himself.
Gondomar conceived his embassy as a sortie in enemy country, and took for his maxim aventurar la vida y osar morir— "risk your life and dare to die"; his opening challenge was his refusal to strike the colours of Spain at his warships' entry to Portsmouth harbour, in which an appeal to the King averted an exchange of cannonfire that would have sunk the ambassador in his vessel. His handling of the unconditional release of the Catholic agitator Luisa Carvajal y Mendoza further established him in James's eyes as a man of unexpected strengths. In 1617 Sarmiento was created Count of Gondomar. The key to Gondomar's success was his relationship with James, whom he brought to admire and like his witty and learned companionship, his candour, within the obvious limits, and his personal integrity. They called themselves the "two Diegos" and drank from the same bottle (Carter 1964:205). A key program for Gondomar at the outset was to block the marriage negotiations between Prince Charles and a sister of Louis XIII, a French counterweight to marital alliances being concluded with the Spanish Habsburgs.
In the matter of Sir Walter Raleigh, it was Gondomar's pressure that lost Sir Walter's head, 29 October 1618. In a moment of weakness James had shown Gondomar the contract under which Raleigh had sailed, the restriction upon attacking Spanish settlements, in order to mollify his objections to an enterprise on which James had set his heart. Raleigh's attack on San Thomé (near modern Ciudad Guayana) was a public violation. His execution kept the peace with Spain. The English could not forgive Gondomar, who in 1618 obtained leave to come home for his health, but was ordered to return by way of Flanders and France with a diplomatic mission.
In 1619 he returned to London, and remained till 1622. The Guyana expedition of Roger North in 1620 seemed to be a repeat of Raleigh's violation of Spanish settlements in the Caribbean, and at Gondomar's insistence North was imprisoned.
The tensest late confrontation was over Count Mansfeld's projected movement of troops raised in England to rescue James's son-in-law Frederick V, Elector Palatine, the "Winter King" of Bohemia. Habsburg Madrid and Brussels were concerned that the French aimed to join Mansfeld's forces and retrieve Artois for France, and the project was let slide.
When Gondomar was allowed to retire, on his return to Spain he was named a member of the royal council and governor of one of the king's palaces, and was appointed to a complimentary mission to Vienna. Gondomar was in Madrid when the prince of Wales— afterwards Charles I— made his journey there in search of a wife. He died at the house of the Constable of Castile, near Haro in the Rioja.
Gondomar was twice married, first to his niece Beatrix Sarmiento, by whom he had no children, and then to his cousin Constanza de Acuña, by whom he had four sons and three daughters. The hatred he aroused in England, which was shown by widespread mockery of an intestinal complaint from which he suffered for years, was a tribute to the zeal with which he served his own master.
Gondomar collected, both before he came to London and during his residence there, a fine library of printed books and manuscripts. Orders for the arrangement, binding and storing of his books in his house at Valladolid appear frequently in his voluminous correspondence. In 1785 the library was ceded by his descendant and representative the marquis of Malpica to Charles III of Spain, and it is now in the Royal Library at Madrid.
In Spanish, Don Pascual de Gayangos wrote a useful biographical introduction to a publication of a few of his letters--Cinco Cartas politico-literarias de Don Diego Sarmiento de Acuña, conde de Gondomar, issued at Madrid in 1869 by the Sociedad de Bibliófilos of the Spanish Academy; and there is a life in English by F.H. Lyon (1910).