I usually use the apt-offline utility to install packages on systems without an internet connection. The advantage of it is that it automatically takes care of dependencies and even works across different systems, e.g. I can use my Mint 17.2 (based on 14.04) to fetch packages for a Kubuntu 13.10. apt-offline uses a three step process.
But first you have to install apt-offline on both the offline and online system. For the online system, you can simply do
sudo apt-get install apt-offline
For the offline system, you may now simply take the package from the online system's cache at
/var/cache/apt/archives/ and install it. All dependencies are already installed (python, apt, less). Should it not work, e.g. due to having a really old system, you can get older versions of the package from here (for current Ubuntu's) or here (for EOL Ubuntu's). Take the Debian archive to the offline computer and install it.
The three step process
At the offline computer, enter the command
apt-offline set --install-packages <package1> <package2> ... <packageN>
This will generate a file called
apt-offline.sig. Take this file over to your online computer. Should you be running an old release, make sure, APT's sources.list point to the old-releases server. In this case, this may require internet for a single
apt-get update, afterwards never again.
At the online computer, cd to the signature file and run the command
this will download the required packages and their dependencies. Take all downloaded files back to the offline computer.
At the offline computer, cd to the folder with the archives in and run
sudo dpkg -i *.deb
Actually apt-offline intends the 3rd step to be done differently, but I found this way to work better.
In this way, you can not only install Mono on an offline computer, but you have a semi-automated way of installing any package on offline debian-based systems.