Fitzherbert was commissioned lieutenant on 20 February 1755 and promoted commander on 14 March 1760, joining the sloop Speedwell 8 and being employed cruising. In February 1761 he commissioned the new Senegal 14, seeing service in the Downs before leaving her in July. During the same year he held the acting command of the frigate Niger 32 for Captain John Bentinck, and cruising in the Channel he captured the Brest privateer Duc de Marizane 12 off Ushant.
He was posted captain on 10 July 1761, joining the Wager 24 for a short time in 1762 and going out to Jamaica prior to her being surveyed at Sheerness and sold off at the end of 1763.
During the 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. This vessel returned home to be paid off in the summer of 1770. From 1774-6 he had the Plymouth guardship Dublin 74, taking part in the summer cruise of 1775.
In the early spring of 1776 Fitzherbert recommissioned the Raisonnable 64, being employed initially as a guardship at Plymouth. He captured the American privateer Dalton on Christmas Eve 1776 and in the ensuing March went out to North America where he participated in the Philadelphia campaign of August-November 1777. In July of the following year he was sent to join Vice-Admiral Lord Howe at New York from Halifax, passing the French fleet two days prior to making a rendezvous. Taking aboard Captain John Orde of the Virginia 28 and many of his crew, their frigate being under repair, he subsequently commanded the Raisonnable in operations off Rhode Island during August 1778.
Shortly afterwards he 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 later that year to return home. In May 1780 he commissioned the newly-launched Belliqueux 64, but also resigned from that command when she was ordered to the West Indies at the end of the year. He subsequently commanded the Alexander 74 from January to April 1783 before she was paid off.
Immediately after the war he commissioned the new Powerful 74, based at Plymouth until she was paid off in 1785.
He was promoted rear-admiral on 21 September 1790 and vice-admiral on 12 April 1794, but later that year died at Stoke Damarell, Plymouth.
His wife, Margaret Langford, predeceased him in 1793. One of his children, Anna Alicia, married Captain George Burgoyne Salt.