As a lieutenant, Losack served on, the flagship of Admiral William Cornwallis, and participated in the enterprise to cut outla Chevrette, a French corvette, of 20 nine-pounders and 350 men, from under the batteries in Camaret Bay, near Brest, on the night of 21 July 1801. Lieutenant Losack and Lieutenant Keith Maxwell were promoted to the rank of commander for their services in this enterprise. In 1801 he was appointed captain and held the temporary command of the 36-gun fifth rate HMS Jason in the spring of 1801. On 1 May 1801 the Jason captured la Dorade, a brig privateer of 14 guns and 51 men. Later that year he was appointed Captain of HMS Helena, an 18-gun sloop. On 5 June 1805 the Helena captured the Santa Leocadia, a Spanish privateer of 14 guns and 114 men.
Post-captain
Losack was made post-captain on 22 January 1806. In 1807 he was appointed Captain of the second rateship of the line. Following the concern in Britain that neutral Denmark was entering an alliance with Napoleon, the Prince George sailed in the squadron in the expedition to occupy the Danish West Indies, with the squadron under the command of Rear-Admiral Alexander Cochrane, who sailed in. The squadron, which included,, and, captured the Telemaco, Carvalho and Master on 17 April 1807.
From 7 September 1810 to July 1814 he was Captain of the fifth rate. On 20 May 1811 he took part in the Battle of Tamatave between a squadron, under the command of Captain Charles Marsh Schomberg, which comprised three English frigates, and the brig-sloop, and three French vessels of superior force. The action marked the end of the final French attempt to operate in the Indian Ocean during the Napoleonic Wars. The surviving participants in the battle qualified for the Naval General Service Medal, which was awarded in 1847 with clasp "Off Tamatave 20 May 1811". There were recriminations among the British squadron, Schomberg praising Astraea and Phoebe but omitting Racehorse and Galatea from the recommendations in his post-battle report. Captain Losack was particularly offended as Schomberg had implied that Galatea's distress signal that was flown during the battle was an overreaction in the face of the enemy, despite her casualties being greater than the rest of the squadron combined. He subsequently requested a court martial to clear any suggestion of cowardice from his name but the Admiralty refused, commenting that they were fully satisfied with his conduct. Historian William James claims that opinion within the Navy was also with Losack and that Schomberg had been excessively harsh in his criticism.
