mongo: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/mongo - from package: mongodb

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/mongo - from package: mongodb-clients

You may install the required package with command apt-get install {package}

Error in other languages:
mongo: bevel nie gevind nie
mongo: comandă negăsită
mongo: comando não encontrado
mongo: commande introuvable
mongo: command not found
mongo: käsku ei ole
mongo: không tìm thấy lệnh
mongo: komanda nerasta
mongo: Kommando nicht gefunden.
mongo: kommandot finns inte
mongo: komut yok
mongo: nie znaleziono polecenia
mongo: níor aimsíodh an t-ordú
mongo: no se encontró la orden
mongo: no s'ha trobat l'ordre
mongo: opdracht niet gevonden
mongo: parancs nem található
mongo: perintah tidak ditemukan
mongo: príkaz nenájdený
mongo: příkaz nenalezen
mongo: команда не найдена
mongo: командата не е открита
mongo: 命令找不到
mongo: コマンドが見つかりません