A swabbing event is being arranged at City Hall where London Assembly staff, including Mr Johnson, are set to take part.
Margot's parent's Yaser and Vicki wrote to the mayor to ask for his support and to look at holding a swabbing event to bolster the stem cell/ bone marrow donor register.
London Assembly member Victoria Borwick said: "I think it is good, particularly at this time of year, to use this forum to raise this sad case and to encourage everyone to think about improving cancer survival rates and to become stem cell donors.
"This follows the letter the family wrote from Team Margot, who are trying, after the tragic death of their own child, to raise the importance of this to Londoners.
"Please everyone sign up and consider. It is only a quick swab test, so can we urge for this to be people's Christmas present to everyone and see how many other lives we can save by getting this message out."
Mr Johnson added: "It's a wonderful idea – we are looking at holding an event with Delete Blood Cancer at City Hall to encourage staff to become stem cell donors."
He then joked: "I'm not sure how much use my stem cells will be, but maybe they will be of use to somebody."
Two-year-old Margot died in October after a rollercoaster year-long fight with two rare forms of leukaemia.
A year earlier Margot had been diagnosed with Acute Lymphoblastic Leukaemia and Acute Myeloid Leukaemia.
Vicki, who grew up in Essington, and father Yaser launched the Swab4Margot campaign under the Team Margot banner, which was backed by celebrities including Stephen Fry and Gary Barlow.
The appeal went viral across the world and led to 50,000 people requesting swab kits from the charity Delete Blood Cancer UK in the first six weeks. Margot was one of 1,800 people in the UK in need of a life saving bone marrow transplant. Visit www.teammargot.com for more information.