Ernest Albert Le Souëf was born at Elwood on 13 September 1869 and was the second son of Albert Alexander and Caroline Le Souëf. Ernest's education varied from his elder brother and instead of going to a boarding school in England, he attended Carlton and Brighton Colleges and the University of Melbourne. In 1888, Ernest followed family tradition and joined the Zoological and Acclimatization Society as the accountant. He was selected to go to England in 1891 to purchase animals and birds under the usual financial limitations and managed to keep the costs so low that he received a gold watch in appreciation. Ernest enrolled in the Melbourne Veterinary College in the following year and qualified as a Bachelor of Veterinary Science in 1895 while working in the evenings. He became the honorary surgeon to the Society.

In 1897, the Western Australian Government intent on establishing a zoological garden, invited A.A.C. Le Souëf to select a site in Perth. On his father's recommendation and in all probability the financial flair he displayed in England, Ernest was appointed First Director. Albert Le Souëf articulated his parental expectations when he informed the council on the selection "I could not spare Ernie were it not for my next son Sherbourne, in his 21st year and could fill Ernie's position at a lower salary".

It was fortuitous that Ernest had inherited the family gift because the financial circumstances in Perth proved more precarious than Melbourne. Staff were a luxury he could ill afford and Ernest was forced to be the resident architect, landscape designer and road constructor. Skilled on hatching and distributing fish for acclimatization purposes, Ernest's responsibilities also extended to travelling vast distances to the hatcheries and bringing animals from the eastern states. The Perth Zoological Gardens were opened on 17 October 1898.

In the midst of the zoological work, Ernest was the foundation member of the Cannington Mounted Rifles in 1899 refining his expert marksmanship. In 1901 he was a commissioned lieutenant in the Australian Army Veterinary Corps and in 1912 was appointed principal veterinary officer (5th Military Division) with the rank of major. At the outbreak of the First World War, Ernest became president of the military horse buying board and in March 1916 enlisted in the A.I.F. for duty with a remount unit. Attached to the Second Light Horse Brigade Headquarters in Egypt, he was wounded in August and repatriated in 1917. After distinguished service, Ernest left the Australian military Forces in 1930 with rank of colonel.

Once he had recuperated, Ernest also began lecturing part-time in agriculture at the University of Western Australia from 1919 and in 1926 was appointed lecturer on a part-time basis in charge of the department of veterinary science. Ernest also founded a museum at the Zoo to accommodate the students for practical anatomy and physiology and provided free veterinary classes to farmers. This generosity and kindness was extended to staff, students and the general community and Ernest was always regarded with affection and appreciation. He retired from the University in 1932.

In the same year, it was apparent that the Zoo's financial problems were insurmountable. In an attempt to resolve them, Ernest recommended that it should be transferred to the State Gardens Board. He then joined the Agricultural Bank as a veterinary adviser for the Margaret River district until his retirement in 1935.

Towards the end of 1937 Ernest suffered a severe stroke during a visit to the Margaret River and died on 27 November. He was survived by his wife, Ellen Grace, daughter of the Reverend F.A. Hagenauer of the Ramahyuck Mission in Gippsland, Victoria who he had married on 20 April 1899, and their five children.