Check out this doc: Use hyperlinks in pop-ups—ArcGIS Pro | Documentation
It highlights what you need to do.
Another option is geodatabase attachments, if you want to image to appear in the popup directly.
And don't forget that you can use the "Image" type in the popup configuration to display the file as well.
Great for local access to images and files that for whatever reason, can't be included in the GDB itself (though attachments are really nice).
One caveat: this only works for desktop GIS. "file:///" links do not translate to web maps.
Thank you very much for the replies everyone it is very much appreciated.
I have got the hyperlink to work in ArcMap without changing anything as you can see below. My problem now is just Pro! Why would it work in one and not the other? Is it just not designed to work in Pro?
I have thousands of data to go through so would rather not have to go through each point and manually put the hyperlink in. Is there a button I can just press which will make my content in these rows hyperlinked?
You can use Arcade and create a popup expression. Check out Replace.
var filepath = $feature.url_field //Example: 'C:/Documents/some_file.jpg' return Replace(filepath, 'C:', 'file://')
And that could just as easily be written to be a UNC path as well.
That will return the link for use in your popup, without requiring you to change the data at all.
However, if you want to permanently update that field, you could also use the same Arcade statement in a field calculation to update the actual attributes.