Firth studerede på [[Central School of Speech and Drama]] og [[Peter Symonds College]] i [[Winchester]]. Hans bofælle på et tidspunkt var skuespiller [[Rufus Sewell]].<ref name=autogeneret2>{{cite web|title=Showcase: Jonathan Firth|url=http://jonathanfirth.com/newyork.html|access-date=2011-07-29|archive-date=28. marts 2012|archive-url=https://web.archive.org/web/20120328080423/http://jonathanfirth.com/newyork.html|url-status=dead}}</ref> Efter eksamen, turnerede han med [[Royal Shakespeare Company]],<ref>{{cite news|title=And the Oscar for best supporting brother goes to... If only Colin Firth's younger sibling had put on THAT wet shirt|url=http://www.dailymail.co.uk/tvshowbiz/article-1358736/Colin-Firths-younger-sibling-Jonathan-wins-Oscar-best-supporting-brother.html|publisher=Daily Mail|access-date=2011-07-29|location=London|first=Polly|last=Dunbar|date=2011-02-21}}</ref> hvor han debuterede i en alder af syvogtyve som Henrik VI (1994).<ref name=autogeneret2 />
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