He began his professional journey with San Lorenzo in Argentina before making a name for himself in international football, particularly during his time with clubs like Universidad de Chile, Cruzeiro, and Botafogo.
His move to Universidad de Chile for $1 million was the largest fee paid by a Chilean club at the time and he also had a stint in Chinese football with Shandong Luneng, spending two years at the height of the local Premier League competition.
In this wide-ranging Q&A with Tribal Football, Montillo discusses his career, the clubs he played for and his opinion on today's big-name South American footballers.
Walter, you played for some of the biggest clubs in South America, what were your most beautiful and challenging moments?
"Every team has its own degree of difficulty, but I believe that playing in Brazil is particularly tough. There is a lot of competition and significant economic power.
"It was a beautiful experience to have had that opportunity. I enjoyed it a lot. Fortunately, things went very well for me, but being a number 10 in Brazil is not easy at all."
You've now moved into player management as an agent, what's your opinion of football in South America at the moment?
"South American football is very rich in raw talent. The biotype here is different from the European one, but it is still a profile that is highly sought after. Personally, I like it a lot because many players still have improvisation in their play.
"There are many names that are in the spotlight, such as (River Plate's Franco) Mastantuono, who, despite his young age, is already a standout."
Speaking of improvisation, you can call Neymar a former teammate from Santos!
"It was great to have him as a teammate and to forge a friendship during those times. It's wonderful to play with players of Ney's quality. I enjoyed it a lot."
And what about your time in China during their football boom?
"I was there for three full years, and for me, it was a very important step in my career. Getting to know other cultures and having to adapt with my family wasn't easy at all. But in terms of sports, we did very well, winning twice."
Before that move to China, you were involved with the national team and played with Leo Messi...
"It was the best thing that happened to me on a sporting level. There is no better feeling than being able to defend your country, at least for me. Being next to Messi was something unique, and I always tried to learn from the players I had by my side."
What about today's Argentina - as Copa América and World Cup champions?
"The national team has been playing very well and has a coaching staff that does not let anyone relax. There is a very good cohesion among everyone."
One of the key men has been Julian Alvarez. What have you made of his decision to leave Manchester City for Atletico Madrid?
"Julian's growth is impressive, and on top of that, he has the ability to perform well everywhere, maintaining an incredible level. I think he will be very successful at Atletico Madrid."
Of course, with those Copa América celebrations over the summer, we had the controversy involving Enzo Fernandez and his French teammates at Chelsea...
"Personally, I think that, at least in Argentina or South America, it's part of the euphoria. I don't think that singing a song in a celebratory environment should be labelled as racist.
"When I played in different places, people said all kinds of things to us, even on social networks, but I never gave too much importance to those things."
Before we leave you, Walter, what are you doing now after gaining so much top-level experience in the game?
"I have my own agency representing players, and I am 100% dedicated to that. I really enjoy being able to accompany the players and help them achieve their goals."