Educated at home in West Newton, Massachusetts, he was mentored by Henry David Thoreau, educated at Antioch College (founded by his father), completed an advanced degree in the botany of the Hawaiian Islands at Harvard College in Cambridge, Massachusetts and was scheduled to teach there, but died of tuberculosis before he could begin, at the age of 23.
Sources
"Massachusetts Deaths, 1841-1915," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:N7VS-D58 : 10 December 2014), Horace Mann, 11 Nov 1868; citing Cambridge, Massachusetts, v 212 p 105, State Archives, Boston; FHL microfilm 960,192.
(parents) "Massachusetts, Town Clerk, Vital and Town Records, 1626-2001," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:FH8V-YHC : 13 July 2016), Horace Mann and Mary Tyler Peabody, 01 May 1843; citing Marriage, Boston, Suffolk, Massachusetts, United States, , town clerk offices, Massachusetts; FHL microfilm 818,095.
en.wikipedia.org/wiki/Horace_Mann_Jr.
"United States Census, 1850," database with images, FamilySearch (https://familysearch.org/ark:/61903/1:1:MDS3-T3D : 9 November 2014), Horace Mann in household of Horace Mann, Newton, Middlesex, Massachusetts, United States; citing family 426, NARA microfilm publication M432 (Washington, D.C.: National Archives and Records Administration, n.d.).
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