A trailer mountable storage container assembly for securely stowing articles on a trailer includes a box, which is mountable to the trailer. A door is hingedly engaged to the box and is positioned to selectively close a front of the box, which is open. The door is lockable to secure contents of the box. A plurality of rails is positioned in the box and is engaged to a back thereof. Each rail extends from proximate to opposed sides of the box and has a channel positioned longitudinally therein. Each of a plurality of hooks is selectively insertable into a respective channel so that the hook is slidably engaged to an associated rail. Items, such as tools, equipment, and the like, can be hung from the hooks and positioned in the box.