In the early part of the Seven Years' War, Captain Fawcett served in Germany as aide-de-camp to Lieutenant-General Grenville Elliott, where he acquired increased knowledge of the military art; and his ardour, intrepidity, and attention to the duties of his situation, were such, that on the decease of Lieutenant-General Grenville Elliott, Captain Fawcett was recommended for the appointment of aide-de-camp to Prince Ferdinand of Brunswick, and he had also the offer of the same appointment to the Marquis of Granby; he chose the latter, and was sent to England with the despatches which gave the account of the victory at Warbourg; on which occasion, King George II. was highly gratified at having the particulars of this engagement related to him in the German language by Captain Fawcett. He was advanced to the rank of lieutenant-colonel in the army on the 25th of November, 1760; and, continuing to serve in Germany, was appointed military secretary to the Marquis of Granby. It is recorded that, in Lieutenant-Colonel Fawcett's character, strength and softness were happily blended together, and to coolness, intrepidity, and extensive military knowledge, he added all the requisite talents of a man of business, and the most persevering assiduity. He was highly esteemed by every officer on the staff of the army, and was the intimate and confidential friend of the Marquis of Granby. He remained on service until the peace in 1763, when he returned to England; and his knowledge of the German language, with the information he possessed from his late office, was the occasion of his being employed by King George III. as commissary to settle the claims made by the Allies against Great Britain for the expenses of the war. In November, 1767, he obtained a company in the third foot guards; and in 1772 he was promoted to the rank of colonel in the army, and nominated deputy adjutant-general of the forces. At the commencement of the American war, Colonel Fawcett was sent to Germany, to negotiate with the states of Hesse, Hanover, and Brunswick, for a body of troops to serve in British pay. In 1776 he was appointed governor of Gravesend and Tilbury-fort; was promoted to major-general in 1777, and nominated colonel of the FIFTEENTH foot in 1778: in 1781 he was constituted adjutant-general of the forces. The rank of lieut.-general was conferred upon this valuable servant of the crown in 1782; in 1786 His Majesty honored him with the riband of the Order of the Bath, and in 1792, gave him the colonelcy of the third, or Prince of Wales's, dragoon guards.