Armstrong came out of retirement in 2009 to ride for the Astana team and finished third in that year's Tour de France. He raced two more years with Team RadioShack with diminishing success before quitting in early 2011.
By then he was the subject of a U.S. federal investigation into doping allegations and more former teammates, notably Tyler Hamilton and Floyd Landis, came forward to specifically implicate the Texan as the ringleader.
Once again, Walsh had been ahead of the game, having published From Lance to Landis: Inside the American Doping Controversy at the Tour de France in 2007.
The federal case against Armstrong was eventually dropped, but the U.S. Anti-Doping Agency persisted in its investigations.
Grilling his former teammates and other close connections, the USADA formally charged Armstrong in June 2012 with using illicit performance-enhancing drugs in "the most sophisticated, professionalized and successful doping program that sport has ever seen."
Walsh, who has won a string of press awards, was finally vindicated as he recounts in his book Seven Deadly Sins: My Pursuit of Lance Armstrong.
For the first time since 2004, Walsh will cover the Tour de France after Britain's Team Sky Cycling team, who won the Tour last year with Bradley Wiggins, gave the journalist exclusive behind the scenes access to the 2013 race.
"I'm really looking forward to it, but I'm looking forward to it because I feel that the Armstrong era has been dealt with and we can start again," said the Irishman.
"We can start tentatively believing in some of what we see, and that's why I'm back."
Not that Walsh believes the doping culture in cycling has been completely eradicated and points to last year's race where Luxembourg's Frank Schleck fell foul of the testers.
"The one certainty is that Frank Schleck wasn't the only guy who doped in last year's Tour de France," said Walsh. "That's absolutely certain.
"Nobody of sane mind would believe that the people who get caught are the only people who dope."
Walsh has spent four weeks in total with Team Sky as they prepared for this year's Tour where Chris Froome, second last year to Wiggins, who will be absent this time, is regarded the favorite.
The journalist has had to soak up some criticism on Twitter that he has become a "PR agency for Sky" but Walsh remains unabashed.
"They say they're clean. I've seen nothing to make me suspicious that they're telling a lie when they say they're clean, though that doesn't mean that things couldn't be happening behind my back.
"The conclusion I've come to given the time I've spent with them is that they certainly don't have an organized doping program within the team."