Within minutes a shooting occurred between rival gangs, including Johnson.
SOCU officers who witnessed the incident were able to provide vital evidence.
After a joint investigation between Lancashire Constabulary and GMP Johnson was arrested and charged with serious firearms offences.
"a leading light in a gang of armed thugs" has today been jailed for firearms offences.
The judge at Manchester Crown Court directed that the custodial period for Leon Johnson, of Annis Street, Preston, should be nine years and eight months.
It comes after Lancashire police carried out a covert investigation into gun crime across Preston.
Operation Decathlon was set up in February 2006 by officers from Lancashire Constabulary's Serious and Organised Crime Unit (SOCU) who targeted 30 year old Leon Johnson.
Johnson was a known member of Manchester's 'Young Gooch Crew' who became a resident on Preston's Callon Estate.
Johnson was found guilty of those offences after a 14 day trial in November, during which SOCU officers from Lancashire Constabulary gave evidence.
Superintendent Dave Brian, from Lancashire Constabulary's Serious and Organised Crime Unit, said: "This is an excellent result for the communities of Greater Manchester but also for the people of Preston.
"The conviction of Johnson should serve as a warning that Lancashire Constabulary will pursue vigorously those who threaten our communities by carrying and using firearms."
In summing up, Judge Gee QC, said: "Johnson was a danger to the public and it was quite clear in the evidence given that he was a leading figure of the 'Young Gooch Crew' and a soldier loyal to this gang.
"He was a significant risk who would have carried out serious harm to other gang members and more importantly to our law abiding members of the public.
"It was for pure good fortune that no members of the public were seriously injured or killed on the day in question."
Detective Sergeant Dave Rimmer, from Lancashire Constabulary's Serious and Organised Crime Unit, added: "I am delighted with the outcome of this trial and also the judge's comments.
"Gun crime is incredibly serious but it is being dealt with robustly by Lancashire Constabulary and this conviction reflects that.
"Today's result also highlights a fantastic joint investigation involving two North West forces who are committed to tackling gun crime."