You can use the mouse to copy Command Prompt text to the clipboard by enabling the Quick Edit option in Command Prompt shortcut properties. What if you don’t use a desktop shortcut to open Command Prompt, and rather run cmd.exe directly? Here is a registry edit which enables Quick Edit option by default for Command Prompt. This works regardless of the methods you use to open the Command Prompt window.
This article was written for older Windows versions. Quick Edit is enabled by default in latest versions of Windows, such as Windows 10.