You can try using fast-export:
cd ~ git clone https://github.com/frej/fast-export.git git init git_repo cd git_repo ~/fast-export/hg-fast-export.sh -r /path/to/old/mercurial_repo git checkout HEAD
Also have a look at this SO question.
If you're using Mercurial version below 4.6, adrihanu got your back:
As he stated in his comment: "In case you use Mercurial < 4.6 and you got "revsymbol not found" error. You need to update your Mercurial or downgrade fast-export by running git checkout tags/v180317 inside ~/fast-export directory.".