What version of Ubuntu Mate are you using?.
I have an old laptop and have to use Ubuntu Mate 14.04 because it would auto suspend during boot using UM 15.10 (I am currently trying UM 16.04 beta on it which apart from one time; went into suspend during bootup!).
Have you checked your settings in Control Center > Look and feel > Screensaver/Power manager?. 
Download link for Ubuntu Mate 14.04.2: