In scenes described by viewers as "bizarre", Newsnight presenter James O'Brien] quizzed his guest on cooking tips before Monbiot marinated the dead animal in lemon juice, fried it, and served two portions - complete with several glasses of red wine.
Monbiot cut off the animal's tail before removing its fur.
Several squirrels, all bought from a farm shop, O'Brien insisted, were used in the segment.
The LBC presenter observed as 52-year-old Monbiot produced an axe, proceeding to butchers it limbs apart and remove any excess fur.
A bowl of lemons was placed on the table in a bid to spruce up proceedings.
Once cooked, both men tucked in to enjoy their 'squirrel surprise'.
Monbiot tweeted prior to his appearance, commenting it was unlikely to be a frequent occurrence that BBC security would be told to let in a man with an axe.
It's not often that BBC security are approached by a producer and told "when a man carrying an axe turns up, let him in."August 27, 2015
In a slightly more ominous post he added: "I'm honing the axe at the moment."
I'm honing the axe at the moment.— GeorgeMonbiot (@GeorgeMonbiot) August 27, 2015
One viewer mused that "hard man" and seasoned presenter O'Brien "looked as though he was about to faint/ gag".
Another suggested the conversation that might have taken place at an editorial planning meeting.
Newsnight news meeting: 'Some big moves in Chinese stocks...' / 'Quiet Duncan, we've go Monbiot to eat a squirrel!' https://t.co/SH291QsNRx— Mike Bird (@Birdyword) August 28, 2015
While a punter weighed in with their own satirical take on proceedings.
And tonight lembit opik will slow roast a lemur on #newsnight— James Lyons (@STJamesl) August 28, 2015
The show's deputy editor clearly knew he was in for a rough ride on Friday.
Leaving the last word to the Financial Times' Janan Ganesh.
Newsnight is unembarrassable— Janan Ganesh (@JananGanesh) August 27, 2015
Watch the full event unfold below