Nyassi began his professional career in 2004 with Gambia Ports Authority of the GFA League First Division. The midfielder remained with the club through the 2007 season, helping the club capture the 2007 Gambian FA Cup, and scoring the game-winning goal in the final. Nyassi's play with the Gambian Under-20 National Team sparked interest among many Major League Soccer clubs. In late 2007 Nyassi was on trial with New England Revolution, the club that signed his twin brother Sainey.
Following a trial spell with newly formed Seattle Sounders FC, Sanna became the third player signed by the club. He was subsequently loaned to Seattle Sounders for the remainder of the 2008 season, prior to Seattle Sounders FC's MLS debut in 2009. On 5 October 2010 Nyassi scored two goals in the final of the 2010 Lamar Hunt US Open Cup Final to help Seattle to a 2–1 victory over Columbus Crew and capture its second straight Cup title. He became the first player in modern history to score multiple goals in the Cup final and was named Player of The Round as a result.
On 24 November 2010, Nyassi was selected by Vancouver Whitecaps FC as their first pick in the 2010 MLS Expansion Draft but he was immediately traded to Colorado Rapids for an international roster slot. On 20 July in a match versus New York Red Bulls, Nyassi scored his first MLS hat-trick and his first three goals for Colorado. His fourth goal of the season came on 29 July against Philadelphia Union. Strike partner Omar Cummings set up Nyassi with a lovely through ball and the Gambian scored with a curling shot to put his side 2–0 up.
Nyassi was left exposed by Colorado in the 2011 MLS Expansion Draft and was selected by expansion side Montreal Impact. Nyassi spent 2 1/2 seasons with Montreal before being traded to Chicago Fire on 29 July 2014 in exchange for midfielder Dilly Duka.
Chicago cut ties with Nyassi at the conclusion of the 2014 season and he entered the 2014 MLS Re-Entry Draft. In December 2014, Nyassi was selected by San Jose Earthquakes in stage two of the draft.
In June 2017, Nyassi sign for Malaysian side Penang FA, a professional association football club currently plays in the 2nd division of Malaysian football, the Malaysia Premier League.
International
Nyassi has represented his nation at various youth levels. He earned three caps playing for the Gambian U-17 national team in the 2005 African Under-17 Championship, scoring one goal against Burkina Faso. He also earned two caps for the U-20 national team in the 2007 FIFA U-20 World Cup. In 2010, he made his debut for the full national team.
