Sinn Féin representative John Finucane was reprimanded by police for indecent behaviour after he was caught urinating in the street earlier this year.
The Lord Mayor of Belfast told the Belfast Telegraph he was "caught short" after attending a social function.
The incident occurred in the Cathedral Quarter area on June 29 this year.
He was issued with a community resolution notice for indecent behaviour.
Mr Finucane told the BBC: "I am of course very embarrassed and deeply sorry about this incident and I apologise unreservedly."
He said he was going home after a social event in the Cathedral Quarter.
"I was caught short and made my way to my office on Castle Street," he said.
"I realised I didn't have my key.
"I tried to find somewhere as discreet as possible off the main street."
He said he was then approached by two police officers.
Mr Finucane is contesting the North Belfast seat in the upcoming general election.
