[1] His grandfather was the prominent lawyer Sir James Mansfield, Solicitor General from 1780 to 1782 and in 1783 and Chief Justice of the Common Pleas from 1804 to 1814.

Mansfield was educated at Royal Military College, Sandhurst, and was commissioned into the 53rd Foot as an ensign in 1835.

[5][6] In 1855, during the Crimean War, Mansfield was appointed military adviser to the Ambassador at Constantinople Lord Stratford de Redcliff, and accompanied him to the Crimea.

[1] He then returned to India and served as Chief-of-Staff during the Indian Mutiny campaign from 1857 to 1859, initially with the local rank of major-general.

His role during the Siege of Lucknow in November 1857 saw his appointment Knight Commander of the Order of the Bath (KCB) in March 1858.