Mint18.2: Mint18.2 has changed it's display manager to LightDM from mdm and as I predicated below this issue went away and the default umask is allowed to be 0002 as intended. For Mint 18.2 no action is required.
To change the default umask in Mint:
Create a script:
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gksu gedit /etc/profile.d/umask.sh
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The effective default umask in Mint is 022 which means that every file you might create on your desktop system say through your file manager will save as 644. Readable to everyone but writeable only to the owner of the file. For most users that's fine but there are circumstances where you might want both user and group to be writeable by default.
In Ubuntu there is a file that among other things controls the default umask: /etc/login.defs
Later on in the file it states:# If USERGROUPS_ENAB is set to "yes", that will modify this UMASK default value
# for private user groups, i. e. the uid is the same as gid, and username is
# the same as the primary group name: for these, the user permissions will be
# used as group permissions, e. g. 022 will become 002.
That file with those settings does exactly what it is supposed to do and sets the default umask to 002. By default new files created by the user save as 664. Writeable to owner and group and readable to everyone else.USERGROUPS_ENAB yes
In Mint that file is identical to Ubuntu's with the exact same settings yet Mint's umask results in 022.
The problem appears to be Mint's display manager - mdm - as seen by this bug report: https://bugs.launchpad.net/linuxmint/+bug/1094990
It looks remarkably like a bug report issued to the gdm display manager 9 years earlier: https://bugs.debian.org/cgi-bin/bugrepo ... bug=314791
I suspect mdm is a fork of gdm. Ubuntu doesn't use mdm or gdm it uses lightdm. In fact if you were to replace mdm in Mint with lightdm the default umask resets itself to 002 without any user intervention.
The traditional methods of changing the default systematically don't seem to work as it appears mdm overrides them. Setting it at /etc/profile.d/umask.sh seems to do the trick.