Dalyell was born in Linlithgowshire, the son of Thomas Dalyell of The Binns, Linlithgowshire, head of a cadet branch of the family of the Earls of Carnwath, and of Janet, daughter of the 1st Lord Bruce of Kinloss, Master of the Rolls in England. He appears to have accompanied Charles I's expedition to La Rochelle in 1628 (to aid the Huguenots during the Siege of La Rochelle) at the age of thirteen. Latterly as a colonel, he served under General Robert Munro and General Alexander Leslie in Ulster.