He was a key member of the senior men's Argentine national basketball team that won the gold medal during the 2004 Summer Olympic Games.

Herrmann began his pro career in the top-tier level Argentine League with Olimpia Venado Tuerto in 1996.

He returned to the Spanish League in July 2009, when he signed a 4-year contract worth €5.05 million euros net income with Saski Baskonia.

Walter scored his NBA career-high 30 points against the Milwaukee Bucks on April 14 and also grabbed a career high 7 defensive rebounds in the same game.

Walter made the NBA All-Rookie Second Team along with his teammate Adam Morrison at the end of the season.

With Flamengo, he won the 2014 edition of the FIBA Intercontinental Cup, and the Brazilian League's 2014–15 season championship.

Herrmann was a key reserve for years on the senior men's Argentine national basketball team.

With Argentina's national team, he won the gold medal at the 2001 and 2004 South American Championship.

[10] Herrmann is married to Spanish doctor Elena, whose father worked in CB Málaga when he played there.