Install Mono

This will install the Mono package from the Debian armhf repository.

Install

Installing Mono on the Raspberry Pi is a simple task. Just use the following command:
sudo echo "deb http://ftp.debian.org/debian sid main" >> /etc/apt/sources.list
sudo apt-get update
sudo apt-get install mono-complete libgdiplus -t sid

If you want to try C# development as well just add monodevelop, too.
sudo apt-get install mono-complete libgdiplus monodevelop -t sid

The installation will take some minutes.

Uninstall

Uninstalling is as simple as installing:
sudo apt-get autoremove monodevelope (only if you installed it)
sudo apt-get autoremove libgdiplus

Hint
libgdiplus removes mono-complete as well.