The writers revealed the family's last name in the pilot episode. It's Wilkerson. In the pilot the name was on Francis' name tag. The writers didn't use the last name for the rest of the series.

We can't find out what's the last name on any of the episodes... But i found out on wikipedia that it's Wilkerson!!!

Yea they haven't told us yet but in the Last Episode "Graduation" On where Francis drops his Name tag from his Work It clearly says "Francis Nolastname" But its not their Last name tho and then later on The teacher calls up Malcolm whom is Valedictorian, but When they try to say his last name but there a was sqeak on the microphone so we r going back to the middle episodes where this kid runs to him and asks him "Hey Malcolm, What is you Last name" then Malcolm Replies"Wilkerson.Why?" and then I forgot what the other guys said so it's Malcolm Wilkerson, Reese Wilkerson, Dewey Wilkerson and at last but not least Francis Nolastname lol jk

His Last Name In The Show Is WilkerSon

It's Wilkerson. They've emntioned it multiple times in the show, but you have to pay really close attention. On one episode, it says Malcolm Wilkerson on his paper.