Allin-Khan received 17,894 votes, giving her a majority win of 6,357.
She was elected in a by-election brought about by Sadiq Khan's departure to become London mayor.
The new MP paid tribute to Jo Cox, who was killed in her West Yorkshire constituency yesterday.
Allin-Khan was chosen by Labour to run as Khan's replacement in May, in a constituency that the party has held since its origin. She was born in Tooting and lives in the area with her family. She works as an A&E specialist doctor at St George’s Hospital and had been a local councillor.
She was running against Dan Watkins, who ran against Khan for the seat in the 2015 General Election, and Liberal Democrat Alex Glassbrook, a barrister, while Elizabeth Jones, who has before stood as an MEP candidate in London, ran for Ukip. Allin-Khan also ran against Alan "Howling Laud" Hope is the Monster Raving Loony Party's representative.