Brendan McCarthy, 50, appeared in court on Tuesday on three counts of causing grievous bodily harm with intent, according to a statement from the UK's Crown Prosecution Service (CPS).
In 2012, McCarthy used a scalpel to split a woman's tongue and also removed another client's nipple. In 2015 he removed a client's ear without using anesthetic, according to the UK's Press Association (PA) news agency.
Authorities launched an investigation after online images showing McCarthy removing the ear prompted complaints, according to the CPS.
The procedures took place at McCarthy's tattoo studio -- Dr Evil's Body Modification Emporium -- in Wolverhampton, England.
A police inquiry began when concerns were raised over McCarthy's practices, said Detective Inspector Chris Fox of the West Midlands Police.
"We found he was performing complex procedures without medical training, outside a clinical environment, without knowing clients' medical histories or psychiatric background, and in the absence of any life-saving equipment should procedures have gone wrong," said Fox in a statement.
Following his arrest McCarthy said that he hadn't broken the law because his clients had given their consent for the procedures to go ahead, according to the CPS.
The tattooist originally pleaded not guilty when he first appeared in court in 2017, reported PA.
Defense lawyer Andrew Smith QC told PA that he would argue that the "particular facts" of the case meant that McCarthy should not be sent to prison.
However a judge ruled that his client's written consent did not provide a defense. CNN has approached Smith for further comment.
McCarthy took his case to the Court of Appeal, but three judges then issued a 12-page ruling in which they argued that McCarthy had undertaken "a series of medical procedures for no medical reason."
"Those seeking body modification of the sort we are concerned with in this appeal invited the appellant to perform irreversible surgery without anesthetic with profound long-term consequences," reads the ruling.
McCarthy is a registered tattooist but does not hold medical qualifications. He is not registered with the UK's General Medical Council, said the CPS.
Rhiannon Jones, Senior Crown Prosecutor at CPS West Midlands, commented on what she said is a landmark case involving body modification.
"This case confirms the existing law that surgical procedures must be carried out by properly trained, qualified and regulated surgeons or health care professionals," said Jones in a statement.
"McCarthy was none of these and as a result his surgical procedures, albeit carried out at the request of his clients, were unlawful."
McCarthy was granted bail pending sentencing on March 21, said the CPS.