Kris Tamiya, 37, of Hucknall Garth, was fined £250 after pleading guilty to not ensuring the health and safety of persons not in his employment.
Hull Magistrates Court heard he reused disposable needles, after cleaning them with hot water and Dettol.
One client told him they had Hepatitis C, but he continued with the tattoo.
'Risk to health'
The court heard Tamiya was also replacing unused inks back into ink pots and allowing cats to walk across the kitchen table while he was applying tattoos.
Tamiya claimed he was tattooing friends to gain experience to allow him to become a professional tattooist, and he did not charge them.
But Hull City Council said it had found sketch books containing tattoo designs, which were marked with prices.
After the hearing, Norma Cottis, from the council's Environmental Health Services, said: "This case highlights he risks to health that the public are exposed to if they use a tattooist or any skin piercer who are not registered and who usually operated from their own home.
"All the legitimate operators within the city have been registered and I would encourage the public to ask to see their certificate which are usually displayed in the premises before having any tattoo or skin pierced."
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