"I've come to the conclusion that it's important for the national team for me to be there, despite my disappointment," the 36-year-old Kahn told a news conference in Munich.
"This can't be about personal vanity.
Coach Juergen Klinsmann settled Germany's long-standing goalkeeping question on Friday when he revealed that Arsenal's Lehmann would be the number one for the hosts for the June 9-July 9 tournament.
Kahn captained Germany to the final of the 2002 World Cup in South Korea and Japan. He was named the tournament's best player, despite a mistake that led to Brazil's first goal in the 2-0 final defeat.
The Bayern keeper said in a statement on Friday that he would consider his international future. After three days of thought, and a 3-0 defeat by Werder Bremen in a Bundesliga match on Saturday, he decided to carry on.
"Juergen has made his decision and I accept it," Kahn said at Monday's news conference, quashing speculation that he might retire. "The team, including Jens Lehmann, will have my full support."
Kahn has played 84 times for the World Cup hosts while Lehmann, also 36, has won 29 caps.
Monday's decision means Kahn can expect to be on the bench at his home stadium in Munich when Germany play the opening match of the World Cup against Costa Rica in Group A on June 9.
One of Klinsmann's first acts as coach after taking over in 2004 was to strip Kahn of the captaincy and inform the Bayern keeper he faced a battle with Lehmann.
The coach originally said he would make a decision at the start of May. That was brought forward to Friday after Bayern complained the indecision was having an effect on Kahn's form.
