An 'out-county' park for Finsbury residents was first proposed in 1850 because of severe overcrowding and the Finsbury Park Act of 1857, the earliest act for a municipal park for London, set aside 250 acres (104ha) of land.
Date : 1857. Finsbury Park Act passed.
HORNSEY. Development from the mid 19th century -
- ac.uk
The park that was finally started in 1866 was 115 acres (48ha) and incorporated the site of Hornsey Wood House (demolished in 1866), grounds and all the farmland enclosed by the railway and roads to the south, west and east - historicengland.org.uk 
Date: August 1869. Park opened.
The structure of the park was designed by Frederick Manable, Superintending Architect to the Metropolitan Board of Works and Alexander Mackenzie, the landscape designer for the MBW, advised from 1868 on the 'ornamental portion of the park'.
# References
Gardeners' Chronicle ii, (1889), pp 184-5 E Cecil, London Parks and Gardens (1907) A MacKenzie, The Parks, Open Spaces and Thoroughfares of London (1898), pp 3-8 J J Sexby, The Municipal Parks, Gardens and Open Spaces of London (1898), pp 309-20 R O Sherington, The Story of Hornsey (1904), pp 89-96 The Victoria History of the County of Middlesex 6, (1980), pp 101-68 Maps John Rocque, Map of Middlesex, 1754 Cruchley, county map, 1828 J Wyld, New Plan of London, 1851 Proposal plan for Finsbury Park, 1857 (MBW/2537/11), (Greater London Record Office) Lithographic plan of Finsbury Park (LCC/Misc.?/202), (Greater London Record Office) Stanford's Library Map of London and its Suburbs, 1877 OS 25" to 1 mile: 1st edition surveyed 1864 2nd edition published 1894-6 Revised edition published 1912-15 1936 edition 1949 edition Archival items MBW and LCC Minutes (LMA) Postcard, photographs, views and cuttings (Bruce Castle Museum)
London Gazette Nov 27 1868 - thegazette.co.uk
London North Mercury And Crouch End Observer Newspaper Archives. December 04, 1903 - newspaperarchive.com
John James Willan (born 1799 in England; died 15 September 1869 at Whitchurch, Hampshire) was an English amateur cricketer who played first-class cricket from 1819 to 1830. He was mainly associated with Hampshire and with Marylebone Cricket Club (MCC), of which he was a member. He made 13 known appearances in first-class matches. He inherited Brownswood (later Finsbury Park) from his father. - douglashistory.co.uk
More history search - connectedhistories.org
Papers on archive.org