Agatha Christie's older brother, otherwise spelt Lewis Mordaunt Miller.  Naturalised British and living in Ashfield, UK in 1891.

Prior to WW1 he had a scheme to run cargo boats on Lake Victoria and he persuaded his sister Margaret to help raise capital. The boat was to be built in Essex and called 'Batenga'. The scheme never finished. When WW1 began the boat was sold to the government and Monty joined the KAR.  Forced from E Africa by debts.

SE - L.M. Miller - Sept. 1907

Land - 1910 - L.M. Miller - Grazing and agricultural, 1221 acres (Farm No. 100) - Uasin Gishu - 26/2/08 - Leasehold under Occupation Licence for 5 to 21 years from 1/3/09 - Registered 8/7/10

Gazette - 4/11/1914 - Appt. - East Africa Transport Corps - To be Lieutenant - L.M. Millar [sic]

Law - L.M. Miller v Rex - November 1912 - …. The Apellant was convicted by Mr Donald Town Magistrate Nakuru for (1) transferring ivory to natives contrary to section 12 of Ordinance No 19 of 1909, and (2) of killing more elephants than he was entitled to under his licence.

London Gazette - 23 Feb 1917 - granted temporary rank for service with the Forces in East Africa - as Captain

Findagrave: Captain, East Africa Transport Corps; veteran of the Boer War and WWI. Brother of author Agatha Christie. Second child and only son of Frederick Alvah Miller, an American gentleman who inherited a moderate fortune, and his British-born wife, Clarissa Margaret Miller née Boehmer. He was named for a family friend, Louis Montant of New York (1848-1877; elder brother of Auguste P. Montant). Monty was born in Morristown, New Jersey, where his parents and sister Madge were temporarily living during an extended visit to the United States; Morristown is about 25 miles from Manhattan, and was then popular with well-to-do New Yorkers seeking a nearby vacation spot. (It is sometimes said that Monty was born in New England; this is incorrect, as New Jersey is a Mid-Atlantic state.) As a result, Monty was an American citizen from birth. The family returned to England in September 1880; the following year, the Millers completed the purchase of the long-term leasehold of a villa called Ashfield in Torquay, Devon. It was there that Monty grew up, and his much younger sister, Agatha, was born in 1890. Monty was not baptized until he was nearing his fifth birthday, on 5 March 1885, at the old parish church of Tor-Mohun, Torquay. His godparents were his maternal uncle Frederick Charles Boehmer, who was then a lieutenant with the Bedfordshire Regiment; Commander Spencer Henry Metcalfe Login, Royal Navy (later a rear-admiral and a Commander of the Royal Victorian Order; Login's own godfather was Brigadier-General Henry Montgomery Lawrence, famed defender of Lucknow); and T.B. Merriam. 

As a youth, Monty enjoyed sailing a one-man boat, and also played cricket. He was enthusiastic enough about wood-working to set up a workshop at Ashfield. He had a dog named Scottie, and also kept white mice. He was educated in England, initially by a governess who spoke French, and later spent three years at Harrow 1894-1896. While he was a good practical engineer and musician, he struggled scholastically, especially over mathematics. In his teens, his parents withdrew him from Harrow and later found a position for him working in the office of a shipyard on the Dart River; this was a common practice amongst middle-class families at the time.

When the Boer War broke out, Monty was determined to contribute. In June 1900, he was naturalized as a British citizen in Ealing (a London suburb, where his paternal step-grandmother, Margaret West Miller--who was also his maternal great-aunt--lived), and that same month was commissioned as a second lieutenant in the 3rd (Militia) Battalion of The Welsh Regiment. Whilst serving in South Africa a year later, he received a commission as a second lieutenant in The East Surrey Regiment, and was stationed in India for an extended period. In January 1904, he was promoted to first lieutenant, backdated to October 1903. He resigned from the East Surreys in February 1907. He next sought to earn a living in the British East Africa Protectorate, and is known to have worked as a professional hunter. Circa 1911, there was some trouble regarding elephant ivory illegally taken and stored with natives. The conviction was reversed on appeal in November 1912, and there does not seem to have been a retrial. His next project was the idea of running a cargo boat on Lake Victoria. He raised money in England and began building the Batenga in an Essex shipyard. Unfortunately, by the time the boat was completed, WWI had broken out, and the Batenga was appropriated by the War Department. Monty returned to Africa and in October 1914 accepted a commission as lieutenant in the East Africa Transport Corps, later promoted to captain. (His sister said he was in the King's African Rifles, which is possible. Many British military service records from WWI were pulped during WWII, Monty’s among them; this makes it difficult to verify details provided in Agatha Christie's An Autobiography.) Late in the war he was wounded in the arm, which became severely infected and permanently compromised his health.

After the Armistice, he worked for a safari service at a place called Mospi, reached via Tanga, Tanzania. But his health deteriorated alarmingly, and he returned to England in early 1922 for medical treatment. For a while, he lived with his mother at Ashfield; later, his family assisted him in making a more settled home of his own at Crossways, Throwleigh, Dartmoor. He recovered sufficiently to buy a motorcycle, and was registered to vote there 1928-1929.

Circa 1929, he and his housekeeper relocated to the milder climate of the French Mediterranean coast. Unfortunately, soon after their arrival, the housekeeper died, and Monty himself was hospitalized. He rallied sufficiently to move to a residence in Marseille, looked after by a nurse. But that September, he died in the British Hospital, Marseille, from a stroke, and was buried at the military cemetery at Mazargues. (The Christie Archive Trust still possesses the title deed to his grave site.)