Having passed his examination on 25 March 1752, Fitzherbert was commissioned lieutenant on 20 February 1755 and promoted commander of the sloop Speedwell 8 on 14 March 1760. Dispatched on a cruise, he enjoyed an early success when capturing a small French privateer and sending her into Mount’s Bay some weeks later. In August he put out of Plymouth with livestock for Admiral Hon. Edward Boscawen’s fleet in Quiberon Bay, and he was later engaged in convoy duty between Plymouth and Portsmouth.
During the early part of 1761 he held the acting command of the frigate Niger 32 for Captain John Bentinck. Cruising in the Channel, he captured the Brest privateer Duc de Marizane 12 to the south-west of Ushant on 12 February, and in the same month he brought the Dunkirk privateer Comte d’Artois 16 into Plymouth. Shortly afterwards he commissioned the new Senegal 14, seeing service in the Downs before leaving her in July.
Fitzherbert was posted captain on 10 July 1761, joining the Wager 24 and being engaged in convoy duty off the south coast before sailing for Cork at the end of 1762 to pick up the trade for the West Indies. Several months later this vessel was paid off, and after being surveyed at Sheerness she was sold out of the service at the end of 1763.
During the years of peace he recommissioned the Adventure 32 in May 1766, going out to Jamaica once more in the following month, and retaining the command for three years before exchanging into the Renown 30 with Captain Hon. George Murray at the end of August 1769. This vessel returned to Portsmouth from Jamaica in July 1770 to be paid off. In April 1773 he joined the Conquestador 60 to receive seamen for the fleet at the Nore, and in November of the same year removed to the Plymouth guardship Dublin 74, taking part in the summer cruise of 1775.
In the early spring of 1776 Fitzherbert paid off the Dublin and recommissioned the Raisonnable 64, which vessel had recently come out of dock, being employed initially as a guardship at Plymouth. He captured the American privateer Dalton 20 from Newberry, Massachusetts, some sixty miles to the west of Cape Finisterre on Christmas Eve, and he took her into Plymouth on 8 January 1777 where her prisoners were initially discharged into the guardship Ocean, and where valuable papers taken from two gentlemen travelling to France were rushed up to the Admiralty. The American privateer, which had plundered nine thousand dollars out of a Newfoundland prize, had initially thought the Raisonnable to be a West Indiaman, and upon realising her mistake had tried to outrun her pursuer over eight hours but struck her colours before an action could commence.
In early February 1777 the Raisonnable went into dock at Plymouth, and after coming out she spent the spring cruising in the Bay of Biscay in quest of more rebel privateers. At the end of June she arrived at New York, having captured two prizes in passage including a valuable ship, the Floridan, bound from Carolina to Nantes, which Fitzherbert sent back to England. She participated in the Philadelphia Campaign of August-November, following which she escorted a convoy of thirty transports to Rhode Island with the expectation of taking General Sir John Burgoyne’s army, which had been defeated at the Battle of Saratoga on 17 October, back to England. With the convention allowing for the repatriation of the troops being repudiated by the Continental Congress, the Raisonnable then sailed for Halifax in early 1778, taking what was reported as an American frigate and two brigs in passage, and where Fitzherbert briefly assumed command of the station in the stead of Captain Sir George Collier.
In July 1778, upon news that the French fleet had arrived off the American coast, Fitzherbert was sent by the then senior officer at Halifax, Captain Charles Fielding of the Diamond 32, to join Vice-Admiral Lord Howe at New York, and two days prior to making that rendezvous he passed the enemy fleet. Taking aboard Captain John Orde and many of the crew of the Virginia 28, which frigate was under repair, he subsequently commanded the Raisonnable in the operations off Rhode Island during August, in the course of which, during poor weather, his command sprung her bowsprit.
Shortly afterwards Fitzherbert exchanged with Captain Henry Francis Evans of the Royal Oak 74, which ship formed part of Vice-Admiral Hon. John Byron’s fleet that sailed for the Leeward Islands at the end of the year. He fought at the Battle of Grenada on 6 July 1779 before leaving her in the following month to return home.
In May 1780 he commissioned the newly-launched Belliqueux 64, which went around to Portsmouth in October and sailed with Rear-Admiral Sir Samuel Hood’s reinforcements to the Leeward Islands in December, during which voyage she collided with the flagship Barfleur 90 and carried away her head. Fitzherbert was subsequently present at the capture of St. Eustatius on 3 February 1781 and on 25 March he arrived at the Admiralty with Admiral Sir George Rodney’s duplicate dispatches announcing the capture of the Dutch island. He subsequently commanded the Alexander 74 from January to April 1783 before she was paid off at Chatham.
Immediately after the war Fitzherbert commissioned the new Powerful 74 at Chatham, being based at Plymouth until she was paid off in May 1786, and thereafter he did not see any further service.
He was promoted rear-admiral on 21 September 1790 and vice-admiral on 12 April 1794, but died later that year at Stoke Damarell, Plymouth.
Fitzherbert’s wife, Margaret Langford, predeceased him in Bath in May 1793. One of his children, Anna Alicia, married Captain George Burgoyne Salt.