The forum was founded in 2009 by Satoshi and Sirius. It replaced a SourceForge forum.
Just wondering if Sirius was the founder then why's Theymos considered the "head" admin of sorts?
If memory serves me correctly Sirius handed the day-to-day management of the forum in or around 2010/2011, and retained ownership and control of the bitcointalk.org domain, and later gave theymos partial control of the domain.
There is also another admin,
Cøbra, whose account was created in 2014. If memory serves me correctly, at one point it was claimed he lived in Japan, and it was claimed that Sirius turned over the administration to both theymos and Cøbra at the same time. I am not sure off hand where these claims were made.