How do I mark messages on Mac as read?

Hi,

If you mean in Messages it will depend on the Address the iMessages was sent to.

iMessages sent to the iPhone Number but appearing on the Mac will not send Read receipts (it will not respond to a "Sync" copy as it were)

The same is true on the iPhone if it is viewing a "Sync" on an iMessage that was addressed to the Apple ID.

I have not tried restricting my Mac version to Receive on at the iPhone Number which might work.

(you would have to leave the Apple ID still logged in though).

8:29 pm Sunday; September 10, 2017

select the unread message(s) (hold the CMD key, or use the shift key when you want all),

then select "Message" from the Mail menu,

then select "Mark as Read".

Selecting the conversation should mark it as Read. If you are wanting to transmit that to the other party, go to Messages/Preferences/Accounts, select the account, and check Send read receipts at the bottom.