General Frederick Rennell Thackeray CB, (1775 – 19 September 1860) was a senior British Army officer.
He was born in Windsor, Berkshire, a younger son of physician Dr Frederick Thackeray. His first cousin was William Makepeace Thackeray.
He entered the Royal Military Academy in Woolwich and became a Second Lieutenant in the Royal Artillery in 1793 but transferred to the Royal Engineers the following year. He served from 1793 at Gibraltar, where he was promoted first lieutenant in 1796, and then transferred to the West Indies in 1797. He took part, on 20 Aug. 1799, in the capture of Surinam under Sir Thomas Trigge. In 1801 he was aide-de-camp to Trigge at the capture of the Swedish West India island of St. Bartholomew, the Dutch island of St. Martin and the Danish islands of St. Thomas and St. John.
In 1807, now a captain, he was sent to Sicily, from where he proceeded to Egypt with the expedition under Major-general McKenzie Fraser, returning to Sicily in September. In 1809 Thackeray was commanding Royal Engineer with a force under Lieutenant-Colonel Haviland Smith, and was detached by Sir John Stuart to make an attack on the castle of Scylla, which Thackeray directed with such skill that, although raised by a superior force of French, the castle was untenable, and had to be blown up.
In 1810 Thackeray was sent from Messina to join Colonel (afterwards General Sir) John Oswald in the Ionian Islands with orders to take part in the siege of the fortress of Santa Maura on the island of Lefkada. The position of the fortress on a long narrow isthmus of sand rendered it difficult to approach, and it was not only well supplied, but contained casemated barracks for a garrison of eight hundred men under General Camus. General Oswald effected a landing on 23 March and the enemy were driven out of their forward entrenchments at bayonet point by the 35th Regiment of Foot. Large working parties were at once sent in and the entrenchment converted into a secure lodgment from which the British infantry and sharpshooters were so able to distress the artillery of the fort that it surrendered. Thackeray was mentioned in general orders and in despatches and received on 19 May 1810 a brevet majority in special recognition of his services on this occasion.