How to change default font in TinyMCE

This is the Answer, It work for me:

STEP # 1:

Look for functions.php in root of your themes directory inside /wp-content/themes/yourtheme/, open it up and add one line after php tag.

add_editor_style(‘custom-editor-style.css’);

STEP # 2:

In the same directory, create a file called custom-editor-style.css with below lines in it

@import url(https://fonts.googleapis.com/css?family=Open+Sans:400,700);
* { font-family: ‘Open Sans’, sans-serif, Arial, Helvetica;}
Go ahead, clear your browsers cache and this is what you’ll see.

Refer link: https://blog.phi9.com/wordpress-editor-and-its-font/

Tony Ngo