Notes for next time I have to set up my external monitor. This is for Ubuntu 16.04 LTS.
I have an Acer VGA monitor, resolution 1600 x 900. The last time I reinstalled Mint, the resolution wasn't detected. Mint 17 couldn't do it, Ubuntu 16.04 couldn't do it; Windows 8.1 worked almost flawlessly (almost, because some of my attempts to fix the problem messed up the display on the windows side).
This was fixed a while ago, but I'm making a record of the solution. I created a small file called set-screen.sh, put it in my home directory, and added it to my startup programs. It would automatically reset the resolution of my external monitor. Hackish, but it worked.
xrandr --newmode "1600x900_60.00" 118.25 1600 1696 1856 2112 900 903 908 934 -hsync +vsync
xrandr --addmode VGA1 1600x900_60.00
xrandr --output VGA1 --mode 1600x900_60.00
The sleep 7 command ensured that my computer had a chance to turn on completely and actually detect the external monitor before running the command to modify the display. I have a pretty fast computer; the recommended sleep time was actually 15 seconds.