Diallo played in twelve countries across four continents: United States, Senegal, Morocco, Switzerland, Saudi Arabia, Malaysia, South Africa, Mali, Germany, Turkey, Sweden and Norway. A prolific scorer almost everywhere he played, he became a regular for the Senegal national team.
Major League Soccer signed Diallo in 2000 and allocated him to the Tampa Bay Mutiny. Diallo exploded on the MLS scene in his first season, combining well with midfield general Carlos Valderrama. Diallo scored 26 goals, the best in the league since Roy Lassiter had 27 in 1996; he was named to the MLS Best XI and was the league Scoring Champion.
However, Diallo is also remembered for the incident on August 16, 2000 in a game against the MetroStars, where he collided with and stepped on Metro goalie Mike Ammann, breaking his ribs, puncturing his lung, and effectively ending his career in the process. While the referee saw the collision as unintentional contact, and while he was neither fined nor carded for the incident, there were those (including Ammann himself) that considered the contact was intentional.
In 2001, with Valderrama traded in mid-season, Diallo had a down year, slipping to nine goals. The Mutiny was contracted after the season and Diallo was selected by the New England Revolution in the 2002 MLS Allocation Draft. He only spent seven games in New England, and was sent over to the MetroStars in a six-player deal on May 24. He had a four-goal game against the Los Angeles Galaxy in only his second match with the team, but his scoring tapered off and the Metros missed the playoffs.
Diallo was sold to Saudi Arabian club Al-Ahli on October 7, 2002, but was dropped in November after failing to impress. His globetrotting ways took him to the former UEFA Cup winners, the Swedish club IFK Göteborg, then Malaysian club Pahang, the Jomo Cosmos of South Africa and finally with Djoliba AC until he retired.
Diallo won 46 caps for Senegal, scoring 21 goals, in a career which saw him represent his country at the African Cup of Nations in 1994. He played for Senegal at the 2007 Beach Soccer World Cup, scoring one goal and receiving one red card.