Lehmann joined Arsenal on 25 July 2003 replacing the recently departed David Seaman. Arsenal's 2003–04 season was historic. The Gunners finished the 2003–04 FA Premier League season unbeaten, becoming the first English club to accomplish the feat in the modern era and only the second to ever have an unbeaten season in English football's top tier, the only other team to accomplish this feat being Preston North End in the 1888–89 season. Lehmann played every match in Arsenal's unbeaten season, though his swashbuckling playing style did lead to occasional mistakes. For example, in the title-winning match at local rivals Tottenham Hotspur, Lehmann pushed Tottenham striker Robbie Keane as the pair waited for a Tottenham corner. He also committed an error that led to the equalising goal in Arsenal's Champions League defeat at home to Chelsea the same season. Despite these mistakes, Lehmann's addition to the Arsenal team had a major effect on Arsenal's ability to finish unbeaten. And Arsenal continued unbeaten until 24 October 2004. At the end of his first season at Arsenal, he had already played 54 times earning himself a Premier League winners medal.