Here we will discuss how to create a new custom master page by using existing master pages in SharePoint Online. Here I am doing the demo in an Office 365 SharePoint Online site. If you have not tried yet about Office 365 SharePoint Online features, then you can sign up for a 30 days trial of Office 365
and start explore now.
Before doing this demo also make sure you have installed SharePoint 2013 designer in your local system or any other system which you can access to make edit. SharePoint designer is a free tool which you can download and install from MSDN site also.
Also you can read:
Follow below steps to create a master page fro sharepoint online using designer:
Open the site in SharePoint Designer 2013 -> Go to Master Page from the left pane in designer
Select the html file of the respective master page that you are going to edit, and right click over it to open it as HTML (it might ask to check out the file, but don’t check out)
Copy the complete content of the opened html file into Notepad and save it in to the local drive as a html file.
Now open your site collection in browser and go to site collection settings page. Click on "Master pages and page layouts" link under "Web Designer Galleries". It will open all the master page and layout
Now add the saved html file to this document library.
Select the content type as "Html Master Page" in the "Add a master page" dialog, and save it
Now go to designer, open the master page gallery, you can find your new master page has been created.
Now you can check out the .html file and can start editing.
Find the below example :
I wanted to hide the left navigation for all of my pages. For that I went to the CustomizedMasterPage.html and checked it out. I made the changes in the html file saved and published. Find the below screenshot.
Preview of master page "Before editing"
After editing the html file in designer
Preview of the master page without the left navigation
Hope this article will be helpful.