That defeat was finally avenged when, on September 12 this year, Jeremy Corbyn was elected leader of the Labour Party. And, as he was 34 years ago, Lansman was at the very heart of this summer's fight by the left to wrest the party from the hands of their opponents.
But Lansman is not a man to rest on his laurels: having helped steer Corbyn to a landslide win in the leadership election, the self-proclaimed Bennite is the driving force behind Momentum, the controversial new "Corbynista" pressure group.
While its proponents say the organisation aims to capture and retain the grassroots enthusiasm sparked by Corbyn's campaign, opponents fear that it aims to purge the party of moderates and is a "party within a party".
Momentum's decision to join with the Stop the War Coalition in lobbying MPs to vote against military action in Syria – and the charges of threats, online bullying and abuse which ensued - have stoked these fears further.
It is, though, Lansman's role in Momentum - and both an acknowledgment of his skills and a deep-seated suspicion about his agenda - which is at the heart of many Labour MPs'
concerns.
During the 1981 campaign, Lansman developed a reputation as a highly capable tactician and a man who, as David Kogan and Maurice Kogan put it in their later account, The Battle for the Labour Party, "does the solid donkey work".
Since his re-entry into Labour politics after the party's defeat in 2010 - he was employed as Michael Meacher's researcher and edits the Left Futures website - Lansman has dusted off the skills he honed in the early 1980s.
He is, says one senior Labour figure, "a highly effective corrosive force". Another suggests that few are better placed than Lansman to ensure that, even if Corbyn does not survive, his legacy lives on with changes to the party's rules which strengthen the hard left's control over policy-making.
Most controversially, Lansman remains an advocate of the central demand of the Bennites: that MPs should be more accountable to their local party members and be forced to face a mandatory re-selection by them every parliament. Having, argues one opponent, "let the cat out of the bag" by publicly raising the issue during the leadership campaign, Lansman has been told to adopt a lower profile.
In a recent BBC documentary about the 1981 campaign, he steadfastly refused to answer questions about Momentum.
Even those who do not share his politics suggest that Lansman can be personally convivial. But, they note, having been in the wilderness for three decades, one of the most powerful men in Corbyn's Labour party has still not shaken his penchant for conspiracy theories and belief that the hard left is the victim of an unending series of stitch-ups.
Someone needs to tell Jon Lansman that, this time, he won.
