Loftus Mansergh was born in Lucknow, Uttar Pradesh, India, on 2 March 1873, the only son of the late Major C S P E Mansergh and Mrs Helen Mansergh of Clifford, Castletown Roche, Southern Ireland. He was educated at the Royal Naval Academy in Alverstoke, Hants where he was a classmate of Murray Fraser Sueter.

He joined the Royal Navy in July 1886.  Promoted to Lieutenant in 1895, in 1899 he joined the 295-ton Twin Screw Torpedo Boat Destroyer HMS RANGER ‘in Command’. This was followed by an appointment to the Submarine Depot Ship HMS HAZARD ‘for Submarines’ in 1902, then to the Submarine Depot Ship HMS LATONA ‘for Submarines’ on 1st January 1903.

After training in HOLLAND Boats and a period in Command of HOLLAND No. 5 he was appointed to the Submarine Depot Ship HMS THAMES ‘for Submarine A1 in Command’ in 1904. During Exercises in the Solent on 18 March 1904 Submarine A1 was run down and sunk by SS BERWICK CASTLE and Lieutenant Mansergh and his crew all died. He is buried in the Haslar Naval Cemetery at Clayhall in Gosport, Hants in a grave sited just forward of the obelisk.