His mother also had influence as the daughter of Colonel Thomas Bladen, and Capel received preferential treatment due to his status from a very young age.

[1][2] In April 1798 he joined the flagship of Rear-Admiral Sir Horatio Nelson, the newly refitted 74 gun HMS Vanguard, under Captain Edward Berry.

Despite the arrival of reinforcements, including ten ships of the line, on 6 June; Nelson remained aboard the jury masted, Vanguard.

[5] Capel was made acting commander of the 16-gun brig HMS Mutine, and was given the honour of carrying the second set of dispatches to London despite the claims of a large number of superior officers.

The first dispatches, carried by Berry in HMS Leander were captured shortly after leaving, which meant that when Capel arrived in London after passage in the brig[2][5] and overland travel, he was the first to bring news of the victory.

[2][5] When war with France was renewed in April 1803 Capel, still in HMS Phoebe was re-attached to Nelson and began the task of observing French naval movements.

[6] Capel was present throughout the ensuing battle of Trafalgar, relaying Nelson's signals to the rest of fleet, and remaining close to the action although not actually engaged with the enemy.

On 4 November, Admiral Collingwood wrote "The extraordinary exertion of Captain Capel saved the French Swiftsure; and his ship the Phoebe, together with the Dougal afterwards brought out the Bahama".

[6] After Trafalgar, Capel returned to England, where he sat on the court martial of Vice-Admiral Sir Robert Calder, before being appointed to the 4th rate 40 gun Endymion.

[6] Capel was once again mentioned in dispatches for his part in the battle and received the ship of the line La Hogue as a command,[2] which he took to the North American station at the outbreak of the War of 1812.

In May 1834, Sir Thomas Capel served as commander in chief of the Far East squadron until July 1837 aboard the fourth-rate frigate HMS Winchester.

Designed by Elizabeth, Lady Wilbraham, in the late 17th century it was built as the Cassiobury Estate's dower house, which still exists in Hempstead Road, Watford.