Merging Photos Libraries

Which system version do you have installed?

The iCloud method described by Niel is the best method on older system versions and if you are already using iCloud Photos anyway. It will result in a true, lossless merging, including all albums, folders, metadata, keywords, titles, captions, faces. And the edited versions and originals will remain stacked as one photo.

With macOS 12 Monterey installed on your new Mac, you can merge two libraries by directly importing one Photos Library into the other, but with limitations. See Apple's support document "Import photos from another library in Photos on Mac - Apple" Support and this user tip:

Importing photos directly from another Photos Library - Apple Community

Importing one library into the other will be useful, if the libraries are nearly identical and only a few photos need transferring. Then you just need to open each of your old libraries on the new Mac to upgrade them to Photos 7 versions, open the larger library in Photos, then use "File > Import" to import the second lilibrary into the larger one. Photos will show all photos for Import, that are missing in the first library and you can add them. The keywords, the faces, the albums will be missing.

Set each as the System Photo Library if they aren't already, upload them to the iCloud, and download the merged library on the new computer.

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