But the 31-year-old was disappointed with his time of 51.90 seconds ahead of the British Championships in Birmingham on 3-5 July.
Williams is continuing his comeback from a four-month ban for failing a drugs test in July 2014.
"I didn't expect to run so slow today... but with a few tweaks I should run a bit quicker next week," he said.
"I'm expecting to run under 50 seconds but it's no good me talking about it, but I've trained hard and I can only do what I can do."
Williams is bidding for a place in the Great Britain team for August's World Championships in Beijing.
But he would have to finish in the top two in the British Championships to be selected and also needs to meet the qualifying time of 49.50sec at an event before Beijing.
Williams only returned to competitive action in May after his ban, which he served after UK Anti-Doping found that he and fellow Welshman Gareth Warburton had inadvertently taken a contaminated food supplement.
Elsewhere at the Cardiff Athletics Stadium, 17-year-old Hannah Brier claimed her first senior women's 100m title in a wind-assisted 11.40sec - just outside the national record of 11.39 sec and all-comers record of 11.27sec.
"I just wanted to concentrate on my start as I've been chasing times a lot and that's not very good," Brier said.
"I'd forgotten that I had to concentrate on my start and I was really happy with that - 11.40 was up there with some of the fastest times I've run this season."
Brier, who added the 200m title on Sunday in 23.62sec, now hopes to be selected for the World Youth Championships, which begin in Colombia on 15 July.
"I've run the qualifying standards quite comfortably, so it's just whether the selection panel now think I'm worthy enough of going out there," she added.
Sam Gordon took the men's 100m title in 10.36sec, with defending champion Dewi Hammond having to withdraw after picking up an injury when winning his heat.
Navid Childs won the triple jump with 14.68m, while Sian Swanson retained her women's triple jump title with 11.06m.
Olly Sheppard was the 3000m champion in 10min 47.52sec, with Vicki Cronin running a personal best of 11.56.23 in the women's event.
Brett Morse successfully defended his discus title with a throw of 58.69m, as did Awen Rosser in the women's event with 39.54m.
There was also success for Cardiff-based pole vaulter Holly Bradshaw, returning to action after 16 months out injured, whose 4.50m led the field and also achieved the mark necessary to qualify for the World Championships.

The ??359m Independent Living Fund pays out an average of ??300 a week, to help people pay for carers so they can live at home and not in a care home.
It was already shut to new applicants for this year and will now shut to new claims permanently, said the minister for disabled people, Maria Miller.
Payments to existing users are to continue until 2015.
Ms Miller said: "An independent discretionary trust delivering social care is financially unsustainable."
But Richard Hawkes, chief executive of disability charity Scope, said the decision to phase out the fund was "bemusing".
"The fund is comparatively very small and is designed to support disabled people to live at home rather than in care homes," he said.
"It's hard to see how phasing out this fund will do anything but narrow down options and push people towards greater dependence on the state."
The phased closure was described as "foolhardy and lacking in humanity" by Labour peer Lord Morris of Manchester.
Lord Morris, who was the first minister for the disabled, said: "This will not save money. If you make it harder for disabled people to live at home, it will cost more because more of them will have to be in hospitals and other places of full-time care.
"It will mean far more of them having to be in institutional care at far greater cost to the taxpayer."
The government said local authorities had a statutory responsibility to provide social care support to their residents.
And it said it remained its priority to safeguard the position of the recipients of the fund.
It said it would carry out a formal consultation next year on how best to continue to support existing users.
Rhys Williams successfully defended his 400m hurdles title at the Welsh Championships in Cardiff on Saturday.
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A fund which supports more than 21,000 people with severe disabilities is to be phased out by 2015.