Frederic Rudolph Lambart was born at Ayot St Lawrence, Hertfordshire, 16 October 1865, the son of Frederick Edward Gould Lambart, Viscount Kilcoursie (later 9th Earl of Cavan), and Mary Sneade Olive. He was educated at home; Mr Cornish's School at Clevedon, Somerset; Eton College; and the Royal Military College, Sandhurst, 1885. He married Caroline Inez Crawley in 1893 (died 1920) and Lady Hester Joan Mulholland (née Byng) in 1922, with whom he had two daughters.

He joined the Army and was commissioned as a Second Lieutenant in the Grenadier Guards, 1885. He served in London and Dublin and then travelled to Canada as Aide-de-Camp to the Governor General, 1891-2. He returned to England and his regiment, 1893-1900, and saw action in the Boer War, 1900-2. He succeeded his father as 10th Earl of Cavan, 1900. He was promoted Captain, 1897, Major, 1902, Lieutenant Colonel, 1908, and Colonel, 1911. He was Adjutant, 1904; second in command of the 1st Battalion, 1906; and Colonel of the 1st Battalion, 1908-12. He held temporary command of the Guards Brigade for manoeuvres and training in the summer of 1913. He retired from the Army to his estate near Wheathampstead, in Hertfordshire, and became Master of the Hertfordshire Hunt, 1913.

On the outbreak of the First World War in 1914, he was appointed to command the 2nd Brigade of the 1st London Division of the Territorial Force and shortly afterwards was sent to France and promoted temporary Brigadier General. He commanded the 4th Guards Brigade, 1914-15, the Guards Division, 1915, and 14th Corps, 1916-17, in the Ypres Salient and the Somme. He was promoted Major General, 1915, and Lieutenant General, 1917. He led 14th Corps in Italy, 1917-18, and assumed command of British forces in Italy, 1918. He was promoted General and given command of the 10th Italian Army later in 1918.

He was appointed Lieutenant of the Tower of London and Commander-in-Chief at Aldershot, 1920-2, and promoted General, 1921. He travelled to the United States as military adviser to Arthur Balfour at the Washington Conference, 1921. He was Chief of the Imperial General Staff, 1922-6. He retired from the Army, 1926.

He and his wife accompanied the Duke and Duchess of York on their world tour, 1927. He was Captain of the Honourable Corps of Gentlemen at Arms, 1929-31, and promoted Field Marshal, 1932. He toured India, 1934, and commanded the troops in the procession at the Coronation of King George VI, 1937.

He died in London on 28 August 1946.

He was created MVO, 1910; CB, 1915; KP, 1916; KCB, 1917; GCMG, 1919; GCVO, 1922; and GBE, 1927.