To do Alt + H M M on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H, M, M.
Command: Merge Cells
This is how to do Alt + H M M 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 M combines the selected cells into one cell without centering. The content from the upper-left cell is preserved; other content is deleted.