He was the founder and first President of Prince’s Golf Club at Mitcham. With his Cambridge University friend Percy Montagu Lucas he provided most of the capital to create new links at Sandwich, now Prince's Golf Club, Sandwich, the land being donated by the Earl of Guilford. The course was designed by Charles Hutchings, the 1902 Amateur Champion, and laid out between 1904 and 1906.
In 1922 he famously acquired control of the large estates of the cash-strapped Duke of Leinster during the latter's lifetime. Fitzgerald had previously sold Mallaby-Deeley his reversionary rights to the estate for a notional consideration, not expecting, as a younger son, to inherit.
Deeley was elected Member of Parliament for Harrow in 1910 and for Willesden East in 1918, resigning in 1923. In 1922 he assumed the additional name of Mallaby, his mother's maiden name, by deed poll and was created a baronet (of Micham Court, Surrey).
- "Deeley [post Mallaby-Deeley], Harry Mallaby (DLY882HM)". A Cambridge Alumni Database. University of Cambridge.
- The Times obituary, 6 February 1937, p. 14
- "Survey of London: volume 36: Covent Garden". British History Online. Retrieved 24 August 2013.
- "Co Kildare Electronic History". County Kildare Library. Retrieved 24 August 2013.
- Hansard 1803–2005: contributions in Parliament by Sir Harry Mallaby-Deeley
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(of Mitcham Court)
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