He was given a six months conditional discharge and ordered to pay £220 costs.
A rally, organised by Cymdeithas yr Iaith Gymraeg was held outside the court building, from 9.30am, before Mr Griffiths was due to appear.
The 56 year-old from Bodorgan, is the second person to face a court sanction for refusing to pay the licence fee.
Another member of Cymdeithas yr Iaith, Heledd Gwyndaf, was sentenced in Aberystwyth last month.
There are over 70 people currently refusing to pay their TV licence fees as part of a campaign to devolve control over broadcasting from Westminster to Wales.
According to a YouGov opinion poll published last year, 65% of people in Wales favour devolving powers over broadcasting to the Senedd in Cardiff.
Speaking ahead of the court case, Williams Griffiths said:
“I’m very concerned about the lack of debate at a Welsh level and how that affects our democracy.
"Every day, broadcasters confuse people by reporting on matters that only affect England but giving the impression that they are relevant to Wales.
"The main broadcasters on all their platforms mainly produce content for England, from England for the benefit of England - we're being drowned in broadcast content that is damaging Welsh democracy. Devolving broadcasting powers to Wales is the only answer to these problems; and that’s why I’m taking a stand."
“This campaign goes to the heart of one of the main threats to our young system of self-government: if people don’t understand who’s responsible for what and what’s being done in their name, how can democracy work?
"The London-based broadcasting system threatens our Welsh democracy. I saw in the 2016 referendum the negative impact of the lack of discussion in the media about how the decision would affect Wales – we really needed more national scrutiny. It's been a massive problem in every referendum and election since devolution."
Aled Powell and the poet and musician Geraint Lovgreen, addressed the rally outside the court ahead of the hearing.
Mr Lovgreen said: "Many thanks to William for his bravery. I, like growing numbers of other people, are also refusing to pay for my TV licence until decisions about broadcasting in Wales are made by the people of Wales.
"The Welsh language and Welsh democracy are seriously suffering as a result of the Westminster-run broadcasting system at the moment.
Controlling our own media in Wales will give us a chance to see the world through Welsh eyes."
Last year, campaign group Cymdeithas yr Iaith presented their proposals for a devolved system of broadcasting.
It claims that tens of millions of pounds extra would be available to invest in Welsh content on TV, radio and online through devolution with control over the licence fee and a new tax on big new media businesses like Netflix, YouTube and Facebook.
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