Following the outbreak of the Civil War in August 1642, he raised a regiment of infantry from his estates in Cornwall and played a prominent role in the 1643 Western campaign, which ensured Royalist control of South West England.

His father's early death meant at the age of six he inherited substantial estates around Plymouth and Falmouth, including Maristow, Bickleigh, Walkhampton and Roborough.

[3] Slanning attended Exeter College, Oxford before entering the Inner Temple in 1628 to acquire the legal training considered essential for members of the gentry at the time.

[6] These were important positions, since the port was commonly used by foreign merchant ships seeking shelter, while Barbary pirates operated in the waters off Land's End.

[11] He was given permission in June to return to Pendennis Castle but in January 1642 was summoned to attend Parliament for allegedly authorising the arrest of the "Five Members", should they try to embark from a Cornish port, a charge he denied.

[15] In October, Slanning relinquished his position as Governor of Pendennis and joined the field army assembled by Sir Ralph Hopton for an unsuccessful attack on Exeter.

[16] The Royalists were very short of basic supplies like gunpowder and ammunition and on 21 February were routed by Parliamentarian troops at Modbury; Slanning's regiment suffered over 350 killed, wounded or captured.

[18] Hopton used the opportunity to reorganise his army, while Slanning brought his unit back up to full strength and they resumed the offensive after the truce ended in April.

These victories ensured Royalist control of South West England but the Cornish foot suffered heavy casualties, including Grenville who was killed at Lansdowne.