Gerrard Scanlon, 53, was arrested after Sky Sports News broadcast footage which appeared to show some of the London club’s supporters abusing Senegalese striker El-Hadji Diouf.
The delivery driver admitted unleashing a foul-mouthed tirade when he was brought before magistrates this morning and fined £425.
He was told to pay further costs of £85 and a £15 victim surcharge, Scotland Yard said.
Scanlon launched his abusive rant during an npower Championship clash at the south-east London ground, known as the Den, on November 18.
Allegations of racism later appeared online before Sky aired the incriminating footage.
Millwall Football Club said it was “appalled” by the clips, while captain Danny Shittu said he was “deeply saddened” by what he saw in the documentary.
The club promised to ban any individuals for life if they were found guilty.
Scanlon, from Enfield, north London, was later accused of using threatening, abusive or insulting words likely to cause harassment, alarm or distress. He was sentenced at Bromley Magistrates’ Court.
Shittu said the club was looking at how it could tackle racism in the aftermath of the abusive episode.