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.

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.

In the ill-starred Bishops' Wars, Astley provided service to the king, and he was involved in the so-called "Army Plot".

[2][4] At the outbreak of the First English Civil War in 1642 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.

[citation needed] At Gloucester Astley commanded a division, and at the First Battle of Newbury he led the infantry of the royal army.

[2] 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.