A series of explosions ripped through London in co-ordinated terrorist strikes in 2005.
The attacks on three London Underground trains and a double-decker bus killed 52 people as well as the bombers, and more than 700 people were injured.
Marking the anniversary on Tuesday, Mr Khan said: “Today we honour the 52 people who lost their lives and more than 700 who were injured on July 7, 2005.
“Our capital will never forget the terrible events of that day, and my thoughts are with all those whose lives were changed forever.
“As we mark 15 years since the attack on our city, I want again to pay tribute to the heroic efforts of our emergency services and transport workers, who ran towards danger to save lives, on that awful day.
“The way that our city responded and stood united in the aftermath of the attack showed the world that our values of decency, tolerance and mutual respect will always overcome the hate of the terrorists.
“Today, we reaffirm our commitment to upholding these values. To those who wish to divide us and spread hatred, we send a clear message that they will never succeed, and that we are stronger together.”
London’s Transport Commissioner Mike Brown also paid tribute.
He said: “We will never forget those innocent victims who lost their lives in the most tragic circumstances 15 years ago.
“We stand united with our colleagues from the emergency services and the city as a whole in remembering them today.
“The resilience of great world cities like ours continues to be tested but Londoners have shown time and time again that our strength lies in our diversity, resourcefulness and spirit of togetherness.”