Mr Benjamin was in the town with fellow far-right activist Milo Yiannopoulos while campaigning to be elected as MEP for the southwest.
Protesters greeted the pair in town with placards, reading: “Sexual harassment = violence”, “Super-callous fragile racist sexist Ukip a***hole”, and “Milo hates everyone including himself”.
And minutes after Mr Benjamin arrived at Truro’s Lemon Quay, the drink was thrown.
It is not clear how much, if any, of the milkshake hit the Ukip candidate.
Supporters of Mr Benjamin pulled the male protester to the ground and violent scuffles broke out.
The protester was later seen walking away.
Earlier this week, West Midlands Police said they were investigating Mr Benjamin in relation to repeated comments he made about raping Labour MP Jess Phillips.
“I wouldn’t even rape you, @jessphillips” he wrote on Twitter. He has since been banned from the social media platform.
Ms Phillips, who is frequently targeted with abuse, spoke about the impact of Mr Benjamin’s comments on the BBC’s Victoria Derbyshire Show.
She said: “I realised that I did what all women do in these situations – I had been putting a brave face on it and pretending that it was all fine and that I could cope.
“It sort of dawned on me that, for four years essentially, this man had made a career out of harassing me. And I felt harassed. I felt, ‘How can somebody say that they would rape me if forced, and be a legitimate candidate in an election?’”
Speaking about the milkshake incident, Devon & Cornwall Police said in a statement: “A milkshake was reportedly thrown over a member of the public by a man wearing black clothing and a white mask.
“Police carried out inquiries but the ‘victim’ did not make themselves known to officers.”
They added: “No official complaint has been received.”
A female protester was also reportedly prevented from throwing kippers at Mr Benjamin.
The milkshake throwing comes a week after another far-right figurehead, Tommy Robinson, whose real name is Stephen Yaxley-Lennon, had two milkshakes thrown over him in two days while also campaigning to become an MEP.