Usage of mono is:
mono [options] program [program-options]
You can run your .exe file as follows:
which is the same as
mono --gc=sgen Log.exe
The difference is the Garbage Collector used. If not specified, then the more conservative BOEHM GC is used. I'm personally using SGEN most of the time because it seems to use much less memory. On the other hand I've seen situations where SGEN on my Raspberry Pi 2 got noticeable slower than BOEHM.
As already mentioned by @exaudio you install the mono runtime environment with:
sudo apt-get install mono-runtime
Or, in case you also want to use mcs to compile C# source files (.cs) into managed executable files (.exe):
sudo apt-get install mono-devel
In case you experience "assembly not found" errors you should go one step further and install mono-complete:
sudo apt-get install mono-complete
But be aware that with Mono 3.2.8 this takes about 166 MB extra memory from your SD card, so be warned and install it only if necessary.