If you previously installed, ran, and created a new account using the geth client and then installed Mist / Ethereum Wallet, your accounts from geth typically show up in Mist / Ethereum Wallet. This is because the keystore files (your encrypted private keys, in JSON format) for accounts created via geth command line and Mist's GUI are stored in the same place.
However, on Linux, the folder may have changed in the past few months. While researching this answer, I found some conflicting reports of where accounts' keystore files were saved on Linux.
In addition, if you were using the eth client (C++, cpp-ethereum) instead of geth, that may also result in accounts being in different places.
Hypothetically, all keystore files are saved at ~/.ethereum/keystore on Linux, but it seems like occasionally this isn't the case.
So, in order to get the account you previously created via geth into your Mist, first locate where your Mist is storing the keystore files. Go to the menu bar and select "Accounts" -> "Backup" -> "Accounts". This will open a folder.
If you already know where your geth keystore files are, or you have a backup, simply copy that keystore file into the folder you just opened via Mist and Mist will read it.
If you have no idea where the keystore files are, you can try doing a search on your computer for either utc--2016 or utc--2015. The full filename for the keystore files looks something like: UTC--2016-04-18T03-24-17.849Z--8688b2e109b93a77578eea43b3ebf6bc4be996f5.
I can confirm on my Mac that searching for utc--2016 displays all my keystore files that were created in 2016 that are not in the (hidden) default location for the keystore files. Explicitly searching the hidden (on Mac it's referred to as "system files") returns search results for the keystore files Mist is reading.
