Nicola Van Renen took a bottle of vodka, valued at £18.
Prosecuting advocate Peter Connick told the court that Van Renen went to the Douglas store on May 5.
She was seen putting the bottle into her handbag, then leaving without paying.
Staff stopped her and when police officers arrived and arrested her, they described her as appearing drunk.
During an interview at police headquarters Van Renen, who lives at Close Quirk, Anagh Coar, Douglas, claimed that she had been confused and had forgotten to pay for the vodka, but said she had intended to return to the store.
However, Mr Connick said that CCTV footage contradicted this and that she had subsequently been banned from Marks and Spencer.
Defence advocate Paul Glover asked for credit to be given for his client’s guilty plea and her admissions to police.
Mr Glover said that it was a relatively low-level shoplifting offence and that Van Renen was embarrassed.
He said that she had spent 12 hours in custody after her arrest and asked for that to also be taken into account.
High Bailiff Mrs Hughes ordered Van Renen to pay the fine and costs at a rate of £20 per week.