Danny Quigley completed the gruelling endurance test in August in memory of his father Colm.
His father, who died suddenly in March 2011, competed in triathlons for Ireland and was a member of the North West Triathlon Club.
Mr Quigley has now raised over £100,000 for local health and wellbeing organisations.
Starting on August 20, the fitness instructor completed an Ironman every day for 10 consecutive days – swimming 2.4 miles, cycling 112 miles and running 26.2 miles each day.
At a full meeting of Derry City and Strabane District Council on Thursday, a motion proposed by Sinn Féin’s Christopher Jackson calling for the freedom of Derry to be bestowed on Mr Quigley passed unanimously.
Other notable recipients of the honour include John Hume, Sir Winston Churchill, former Catholic and Church of Ireland bishops Edward Daly and James Mehaffey and treble-winning Derry City football boss Jim McLaughlin.
Over his 10-day challenge people from across the city and district joined him for sections of his challenges, with others lining routes to cheer him on.
Mr Jackson said: “Danny Quigley’s incredible feat of ten Ironman races in ten consecutive days caught the imagination of all of us”.Mayor of Derry City and Strabane and DUP councillor Graham Warke said Mr Quigley “has brought a city together” and said it was something “that was very much needed during such a difficult period”.
Mr Quigley will be officially bestowed with the freedom of the city at a later date in a formal ceremony with the mayor.