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, 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.  
sleep 7

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.