To do Alt + h m a on Mac in Excel, download and install Accelerator Keys, then press Option, followed by h, m, a.
Command: Merge Across
This is how to do Alt + h m a on Mac in Excel with Accelerator Keys, the native ribbon and alt key shortcut solution for Mac users who get more done with Excel.
Alt + h m a merges cells in each row of your selection separately. Unlike Merge & Center, this keeps each row as a separate merged cell.