Cabinet Member for Housing and Communities, Cllr Lynda Thorne, said:
"It's almost one year on since the November 23 deadline for all landlords with properties in Wales to become registered and yet there are landlords and agents still out there who believe they can operate outside of the law.
"We are taking action and we will find out about those individuals who are not complying. As well as court action and potentially a large fine, landlords and agents could face rent penalties and restriction on re-possession of their property as a result of non-compliance.
"There is no excuse for a commercial agent not to be registered and licensed. I urge any landlords who use a commercial agent to check they are Rent Smart Wales compliant to ensure they are carrying out management duties legally."
Yvette Phillips, trading as estate agent R Miles Scurlock of Milford Haven, Pembrokeshire, failed to submit a completed licence application or register her rental properties.
She was issued with a Fixed Penalty Notice in June this year but she failed to pay, submit a licence application or register the properties despite further reminders.
Cardiff Magistrates fined Mrs Phillips £4,600 for three offences under the Housing (Wales) Act 2014, ordered her to pay court costs of £671 and a £170 victim surcharge.
Self-managing landlord Damian Cross of Rhodfa'r Gwagenni, Barry was also found guilty of failing to become licensed for letting and management activities for a property at Beaconsfield, Romilly Road, Barry.
Mr Cross had registered the property but not applied for a licence. He failed to pay a Fixed Penalty Notice for his non-compliance, or submit a licence application or appoint an agent.
He was fined £660 for two offences under the Housing (Wales) Act 2014, and ordered to pay court costs of £543 and a £33 victim surcharge.