Microsoft had a research version of the CLR called Rotor (2002) that predated Mono (2004). Rotor built for Windows, FreeBSD, and macOs, albeit with a not-very-open license.
When Mono came along, the internal position at Microsoft was surprisingly positive. There was a dev slide deck that went into Mono in some depth. And a telling slide that said it wasn't a threat because the performance wasn't competitive at the time.
https://en.m.wikipedia.org/wiki/Shared_Source_Common_Languag...
https://en.m.wikipedia.org/wiki/Mono_(software)
