The first alpha versions of Mathics were done in 2011 by Jan Pöschko. He worked on it for a couple of years until the v0.5 release in 2012. At that point, it had the first iteration of 386 built-in symbols. Currently, there are over 1,300, and even more when Mathics3 modules are included.
After that, Angus Griffith took over primary leadership and rewrote the scanner and parser to make it much closer to the stage it is in now. He and, later, Ben Jones worked on it from 2013 to about 2017 for the v1.0 release. Towards the end of this period, Bernhard Liebl worked on this, mostly focusing on graphics.
A docker image of the v.9 release can be found on DockerHub.
Around 2017, the project was largely abandoned in its largely Python 2.7 state, with some support for Python 3.2-3.5 via six.
Subsequently, around mid-2020, it was picked up by the current developers and the project was renamed from Mathics to Mathics3. A list of authors and contributors can be found in the AUTHORS.txt file.