Legge taught at Sydney Boys High School from 1886 to 1890 when he stood down to practice law, being admitted to the bar in New South Wales on 6 March 1891.

In 1899 Legge delivered to the United Services Institution a proposal for a 90 000 strong "Australian Defence Force", based on compulsory military training of men aged between 20 and 25.

[4] With the outbreak of the Second Boer War in October 1899, Legge was appointed to command an infantry company, with Lieutenant William Holmes as one of his subalterns, and promoted to captain in December.

Then on 1 September 1904, he was promoted to major and given the post of Deputy Assistant Adjutant General at Second Military District Headquarters in Sydney.

With the advent of the second Deakin government, Legge began to work directly under the Minister for Defence Thomas Ewing as chief planner for a complete refoundation of the Commonwealth's land forces on the basis of compulsory military training.

With the rank of temporary lieutenant colonel, Legge was appointed Quartermaster General and a member of the Military Board in January 1909.

On 1 May 1914, Legge was appointed Chief of the General Staff, with the rank of full colonel, but he did not sail for home until July 1914.

When Bridges was fatally wounded by a Turkish sniper in Monash Valley in May 1915, Legge was the natural choice of the Australian government to succeed him as both commander of the 1st Division and of the AIF.

The Australian field commanders, Colonels Harry Chauvel, James Whiteside McCay and John Monash, were disappointed at being passed over by an officer who was their junior, with no recent combat experience, and protested to Generals William Birdwood and Hamilton.

He arranged for the colonels to be promoted to brigadier general in line with their British counterparts, with seniority back dated to their assumption of brigade command.

Like his predecessor, Legge felt that the proposed assault on Lone Pine would be costly and most likely futile unless the high ground above it was first seized.

Birdwood held that only an attack on a key position like Lone Pine would cause the Turks to divert troops from opposing the main effort.

Birdwood seized on an opportunity to remove Legge from the scene when Brigadier General McCay, about to take command of the 2nd Division, which had begun forming in Egypt, broke his leg and was evacuated on 11 July 1915.

Legge won the admiration of many for the quiet and good humoured way he handled the situation, remaining on board with the last 400 men, who were eventually transferred to the hospital ship Neuralia.

From mid-October, Legge occasionally acted as corps commander whenever Major General Alexander Godley was absent.

The division soon became caught up in raids, and casualties were high because of Birdwood's insistence on manning the front line more densely than recommended.

In the next few days, Legge strove to get another attack ready, all the while under tremendous pressure from the enemy, who shelled the 2nd Division's positions nonstop, compelling postponement first to the 2nd and then to 4 August.

As Chief of the General Staff, Legge's role was dealing with politicians in Australia, and providing reinforcements for the AIF overseas.

In January 1920, Legge was appointed to a committee chaired by Harry Chauvel to examine the future structure of the army.

[7] By 1923 cottages for a manager and workmen were being built at Cranleigh, farm machinery including a tractor had been purchased and a concrete weir across the Ginninderra Creek was under construction.

The decision[7] contains a detailed account of the life of Legge and his reasons for remaining in the Australian Capital Territory in his retirement.