 On the formation of the New Model Army in 1645, Packer became a captain in Fairfax's regiment of Horse.He was with Fairfax at the siege of Colchester in 1648 and served as a major in Cromwell's regiment in the Scottish campaign of 1650.During the Rule of the Major-Generals (1655), he deputised for Charles Fleetwood as military governor of Hertfordshire and Oxfordshire.