Spanish naval officer. Teniente de navío Pedro de Mesa y Castro served two terms as comandante gobernador of the Spanish settlement at Puerto de la Soledad (Port Louis) in command of the corvette San Gil, with José ALDANA, a future governor of the Falklands as his second-in-command. He first took over from Ramón de CLAIRAC on 25 May 1786 and was relieved by Clairac again on 15 May 1787. He returned for a second term as governor, taking over from Clairac on 10 April 1788, serving until 16 May 1789 when he was relieved once again by Clairac. On 10 April 1788 he submitted a manifest, in which he stated that the establishment, including the crew of the San Gil, consisted of 13 officers, 28 soldiers forming the garrison, 105 seamen and 22 convicts. The livestock comprised 2180 cattle, 116 horses and 66 geese. His activities, as shown in his reports to the Viceroy were similar to most of the preceding governors, namely supervision of the welfare of the civilians and military personnel, proper functioning of the presidio (convict colony), repair to buildings and the maintenance of vessels. In particular he had to attend to the state of preparedness of the ships assigned to carry out the prescribed reconnaissance of the coastal areas under his command to see that no foreigners, especially the British, had attempted to breach his territorial jurisdiction. Mesa personally undertook the yearly visit to Port Egmont and produced a plan of the harbour. In March 1787 he sent a special report to Viceroy Marqués de Loreto concerning the poor showing in the number of cattle that year. During the months of June, July and August the previous year he had been obliged to provide the full ration of fresh meat to everyone in the settlement, because there were no adequate reserves of salt meat for the full year. In this situation, and so as not to increase the slaughter of the cattle, he had supplied meat from the San Gil, keeping back only sufficient for her return to Montevideo. As a result the cattle had not increased at the usual rate, while there had been a great loss of calves in the unusual cold winter.

On 10 April, as he began his second tour as governor, Mesa gave the Viceroy a fairly complete description of the state and condition of the settlement at that time. He did this in the form of a detailed exposé intended to be read in conjunction with a table showing at a glance the establishment of the settlement and a list of 36 buildings existing at Puerto de la Soledad at that date. Each numbered item in this list included the use of the building and the material it was made of, either stone or turf. On 12 April 1789 Mesa wrote to the Viceroy, mentioning the baptism of two Pampa Indians at Puerto de la Soledad and the sad state of the church, which was built of turf. On 16 May 1789 Mesa was relieved once again by Clairac.