Macbean was born 28 June 1725, the son of the Reverend Alexander MacBean of Inverness and his wife Marjory, daughter of John MacBean, the son of William MacBean of Inverness; although some have cited him as the son of John MacBean Sheriff Clerk of Inverness who was a brother to William MacBean of Faillie in Strathnairn; this is not correct as that man's daughter 'Margaret' was born three years after Rev. Alexander married his wife Marjory. He entered the Royal Military Academy, Woolwich, as a cadet-matross on 16 July 1743, and passed out as a lieutenant-fireworker on 25 March 1745. Three weeks after his appointment in 1745 Macbean marched with the artillery from Ghent, and had command of two guns at the battle of Fontenoy on 30 April 1745. On the news of the Jacobite rising in Scotland, the four companies of artillery of the Duke of Cumberland's army were sent home. Macbean joined Cumberland's army at Lichfield, and served at the siege of Carlisle in December 1745. In the following summer he went back to the Low Countries, and made the campaigns of 1746–48, commanding the battalion of the 19th Regiment of Foot at the battle of Roucoux, and a detachment of two guns at the battle of Val.