Companies House documents obtained by the Standard show that Mr Johnson has owned 33 per cent of the shares in the TV company since it was founded in September 2006.
On 28 January, Mr Johnson did declare in the MPs' register an unspecified income from Finland Station for the making of The Flame Of Rome but made no other reference to it.
Finland Station has yet to produce trading accounts, so it is impossible to state how much Mr Johnson's stake in the company he set up with Barnaby Spurrier and John Nicholas is worth.
This is the second time that Mr Johnson has breached the rules on his finances. Earlier this year he was reprimanded by the Electoral Commission for failing to declare correctly £45,000 in donations.
After being contacted by the Evening Standard, a spokeswoman for Mr Johnson, said: "Boris Johnson has declared a payment from Finland Station of £30,000 as writer and presenter of a documentary. This is the only income he has received from the company."
Mr Johnson's other outside interests include £250,000 per year from the Daily Telegraph, after-dinner speaking which recently made him £50,000, and book contracts worth as much as £40,000.