The helmet was bought for £7 at a car boot sale 12 years ago but is expected to fetch up to £7,000.
The helmet was one of six used as props by Lucas to pitch his original film idea to movie executives.
Other items on offer at the props sale include original Goons scripts and the moon buggy from Diamonds Are Forever.
The James Bond buggy had fallen into disrepair but has been restored by its current owner and could fetch up to £30,000.
The stormtrooper helmet was also used in the original Star Wars film and follow-up The Empire Strikes Back.
A moustache worn by Charlie Chaplin in The Great Dictator is also up for grabs, along with a truncheon he used in Easy Street and a cane from another of his films, Modern Times.
More recent memorabilia on offer includes the rhinestone-studded tracksuit and knuckle-duster ring worn by Sacha Baron Cohen's TV character, Ali G.
The Goons scripts date between 1956 and 1957 - one of the scripts is from The Mighty Wurlitzer, one of Spike Milligan's favourite shows.
Viewing takes place on between Sunday and Tuesday December at Christie's in Kensington, London. The sale begins on Tuesday afternoon.