Sadiq Khan claims hospitals are overcrowded and other vital services are hampered by staff shortages.
Since the last winter coronavirus wave, London hasn't had this problem.
The capital has witnessed 65,525 new confirmed cases in the last seven days, a new high.
The mayor's office said frontline employee absences are ‘challenging'.
A major crisis occurs when emergency services are strained to the point of endangering human life, vital services, the environment, or national security.
A central committee with a government representative will oversee more inter-service coordination.
It lasted from January 8 to February 26.
Hospital admissions in London have risen by 29% in the last week and are expected to continue.
‘The spike of Omicron cases across our capital is quite worrying, thus we are declaring a major incident again due to the threat of COVID-19.
‘The Omicron variant has swiftly become dominant, with instances growing and COVID-19 patients returning to our institutions.
It's right that London's major agencies work closely together to minimise the impact on our city, including assisting to protect the vital vaccination programme.
‘We know that the vaccine is our best viral defence. London now has more immunisation clinics than at any time during the outbreak.
Please arrange an appointment or visit one of the many walk-in locations across London as soon as possible.
The news comes after the Fire Brigades Union warned that staff shortages have left up to a third of London's engines unstaffed this week.
The London Ambulance Service has advised people to only dial 999 in an emergency.