Mono was started by Miguel at Ximian, and most of the Mono code was owned by Ximian. Ximian was bought by Novell and with it, all the Ximian-owned Mono code. (I don't remember if a CLA was required before or after Novell bought Ximian.)
The Novell-owned IP was either sold or licensed to Xamarin later.
The Novell-owned IP was either sold or licensed to Xamarin later.