Sosnowski made his professional debut in July 1998, defeating Czech fighter Jan Drobena via first-round knockout in Outrup, Denmark. He won his first 19 fights, 12 wins by knockout.
In July 2001, Sosnowski fought for his first title belt against Canadian Arthur Cook in Budapest, Hungary, for the vacant WBC World Youth heavyweight title. Sosnowski, who was clearly ahead on points all scorecards, was knocked out by Cook in the ninth round to take the title.
In July 2008, Sosnowski lost to journeyman Zuri Lawrence, who had taken the fight on late notice but in November 2008 he defeated British fighter Danny Williams when Sosnowski himself was the underdog and he had taken the fight at late notice. He won by technical knockout after 1:17 of the 8th round.
He fought Francesco Pianeta for the EBU-EU heavyweight title on 4 April 2009 with the fight going to a draw after twelve rounds.
He was subsequently lined up to face heavyweight Prizefighter series winner and former Olympic gold medallist, Audley Harrison for his first defence of the title, with the fight being scheduled for 9 April 2010. He cancelled this fight for a shot at Vitali Klitschko for his WBC title.[2] Sosnowski lost to Klitschko by 10th-round knockout after being worn down by shots to the head throughout the fight.