The campaigner is currently in critical condition and a vigil at King’s College Hospital is due to take place on Monday afternoon.
Here’s what we know so far:
Who is Sasha Johnson?
Ms Johnson is an activist and campaigner, who rose to prominence during last year’s Black Lives Matter protests in the UK.
She is a member of the Taking the Initiative Party (TIIP), a political party that was registered with the Electoral Commission nearly four years ago.
According to its website, the party supports a "coalition leadership" and Ms Johnson is one of eight people on its executive leadership committee.
Her page on the TTIP website says Ms Johnson has an “immense passion for implementing change and justice and has been actively supporting the eradication of injustices in society by attending and leading protests.
"Although not everyone may agree with her methods, she is not afraid to stand up for what she believes in,” it states.
"She is also committed to feeding the homeless and has personally been involved in delivering food and groceries to families that are in need."
Ms Johnson works in activism and community support and studied at Oxford University, where she got a first in social care.
The mother-of-three was one of the organisers of the first-ever Million People March to protest against systemic racism in the UK.
During the demonstration, she said she hoped the movement would "empower the community to strive for better".
"As a people, we’re not going to stop until we have equal rights and justice," she said.
"Our message is listen to us, hear our words, we want sustainable and tangible change.
"We don’t just want tokenistic promises, we don’t want it to come from a hegemonic standpoint. We want it to be for the people."
Ms Johnson has also reportedly advocated for reparations to be given to black people in the form of tax breaks due to the legacy of slavery and has been a supporter of the campaign to remove the statue of Cecil Rhodes from Oxford University.
What happened to Sasha Johnson?
Police and paramedics were called to Consort Road, Peckham shortly before 3am and found Ms Johnson injured.
Officers believe a house party was taking place nearby and that witnesses might have seen the shooting.
Det Ch Insp Jimi Tele said: "This was a shocking incident that has left a young woman with very serious injuries. Our thoughts are with her family who are being provided with support at this terribly difficult time."
He added: "If you saw anything suspicious in the Consort Road area in the early hours of Sunday morning, or if you have heard information since that could help detectives, it is crucial that you get in touch."
Police have said at this stage there was no evidence the attack was targeted.
What are the latest updates?
The campaigner’s friend Imarn Ayton, told BBC News that the young mother had successfully undergone surgery and her parents are with her in hospital.
Asked about the circumstances, she said: “As far as we are aware, she was at a party.
“There was a rival gang that may have heard about someone being in that party that they didn’t feel quite comfortable with or trusted and so they resorted to driving past and shooting into the garden, and one of those shots obviously hit Sasha Johnson.
"I think it’s more to do with gangs or gang violence or rivalry or some type of dispute between two different groups, and the incident was not intended for Sasha as far as we are aware, as I am being told by closer friends and family."
Detectives from the Met’s Specialist Crime Command are continuing to investigate the scene and the surrounding area and are pursuing a number of lines of inquiry.