Johnson, speaking at a Business Backs Education summit at City Hall, also warned that the UK capital faces a "very great skills shortage".
The Office for National Statistics said that unemployment fell by 146,000 to 2.02 million in the three months to July, pushing the UK unemployment rate down to 6.2% – the lowest level since November 2008.
But the research body also revealed that the youth unemployment rate (for 16 to 24 year olds) was more than double at 16.6% in the three months to July across the UK and London had a total jobless rate of 6.9% over the same period.
The comments coincided with the launch of research from Business Backs Education which found that top UK companies only spend a fraction of their corporate social responsibility budget on education.
The report, which analysed the 26 UK companies featured in the 2013 Fortune Global 500, found that the firms spend $3.25bn (£2.02bn, €2.56) per year on CSR.
But the study identified education-related expenditure only amounted to $307m, which equates to 9% of the total UK corporate CSR budget.
The Business Backs Education campaign challenges business to commit 20% of their global CSR spend on education by 2020, to bring business in line with government and NGO education spending targets.