Set PATH for a systemd unit

The simplest answer is to set the PATH as part of your ExecStart command in the systemd Unit file. For example, if you currently have

ExecStart=/bin/mycmd arg1 arg2

then change it to

ExecStart=/bin/bash -c 'PATH=/new/path:$PATH exec /bin/mycmd arg1 arg2'

The expansion of $PATH will be done by bash, not systemd. Alternatives such as using Environment=PATH=/new/path:$PATH will not work as systemd will not expand the $PATH.