To do Alt + H M on Mac in Excel, download and install Accelerator Keys, then press Option, followed by H and M.
Command: Merge & Center Menu
This is how to do Alt + H 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 opens the Merge & Center dropdown menu, giving you access to cell merging options. From here you can Merge & Center (C) to combine selected cells and center the content, Merge Across (A) to merge cells in each row separately, Merge Cells (M) to combine without centering, or Unmerge Cells (U) to split previously merged cells back to their original state.