Born about 1551, he was the son of Alexander Carleill, citizen and vintner of London, by his wife Anne, daughter of Sir George Barne.

Afterwards, he went with one ship and a vessel of smaller size to La Rochelle to serve under Henri, Prince of Condé, who was about to send supplies to the town of Brouage, then besieged by Charles, Duke of Mayenne.

By the interest of his stepfather (the old DNB says "father-in-law", a term that meant "stepfather" in the 16th century; Carleill was unmarried), Sir Francis Walsingham,[3] Carleill received £1,000 subscribed at Bristol for an attempt to discover the coast of America lying to the south-west of Cape Breton Island, and proposed to the Russian merchants to raise more in London, and to settle one hundred men in their intended plantation.

In 1584, Sir John Perrot, Lord Lieutenant of Ireland, appointed Carleill commander of the garrison of Coleraine and the district of Route, County Antrim.

Carleill was captain of the Tiger and in charge of the troops that were conveyed which led to the plundering of Santiago in the Cape Verde, after which they then seized Santo Domingo, captured Cartagena de Indias, and St. Augustine.

On 10 June 1590, he wrote to Lord Burghley, requesting a commission from the queen to seize for lawful prize any goods which might be found in England belonging to Spanish subjects, and complaining of his monetary losses in her service.