He came from an established Norfolk family, and was born at Melton Constable. His first experiences of war were at the age of 18 when he joined the Islands Voyage expedition in 1597 under the Earl of Essex and Sir Walter Raleigh to the Azores. In 1598 he joined Maurice of Nassau and Henry of Orange in the Netherlands, where he served with distinction, and afterwards fought under Frederick V, Elector Palatine and Gustavus Adolphus in the Thirty Years' War. He was evidently thought highly of by the States-General, for when he was absent, serving under Christian IV of Denmark, his position in the Dutch army was kept open for him. He married a Dutch woman, Agnes Impel, who bore him two sons and a daughter.
Returning to England with a well-deserved reputation, he was in the employment of Charles I in various military capacities. As "Sergeant-Major-General" of the infantry, he went north in 1639 to organize the defence against the expected Scottish invasion. Here his duties were as much diplomatic as military, as the discontent which ended in the Civil War was now coming to a head. In the ill-starred Bishops' Wars, Astley did good service to the cause of the king, and he was involved in the so-called "Army Plot".
At the outbreak of the First English Civil War in 1643 he at once joined Charles, and was made Major-General of the Foot (infantry) - the cavalry was under the command of his former student Prince Rupert. His characteristic battle-prayer at the Battle of Edgehill has become famous:
Astley was loyal supporter of the Crown throughout the First Civil War, while his own region of East Anglia was strongly Parliamentarian. His opposite number at in the Parliamentarians was Philip Skippon, another Norfolkman. At Gloucester Astley commanded a division, and at the First Battle of Newbury he led the infantry of the royal army. With Ralph Hopton, in 1644, he served at Arundel and Cheriton. At the second Battle of Newbury he made a gallant and memorable defence of Shaw House. He was made a baron by King Charles, and at the Battle of Naseby he once more commanded the main body of the infantry. He afterwards served in the west, and with 1,500 men fought stubbornly but vainly at the Battle of Stow-on-the-Wold (March 1646), the last pitched battle of the First Civil War. He surrendered to the Parliamentarians with the words "Well, boys, you have done your work, now you may go and play - if you don't fall out among yourselves."
His scrupulous sense of honour forbade him to take any part in the Second Civil War, as he had given his parole at Stow-on-the-Wold; but he had to undergo his share of the discomforts that were the lot of the vanquished royalists he was imprisoned initially but able to retire to Maidstone. He died in February 1652. The barony became extinct in 1668.