Social worker Tommy Gale won the seat with 882 votes.
Don Williams, who was Swiss Cottage councillor for the Conservative Party from 2002 to 2018, came second with 766 votes.
The by-election on June 1 was triggered by the resignation of its Labour councillor Will Prince, who is moving to America to work.
South Hampstead only became a ward in the 2022 Camden elections.
Most of its area was previously in Swiss Cottage ward, which was abolished at the same time, although sections of Kilburn and West Hampstead wards have also been incorporated making it a very sought after seat.
Liberal Democrat Patrick Stillman came third with 531 votes.
Lorna Russell, a former Green Party councillor for Fortune Green who lost her seat to the Liberal Democrats in 2022, received 295 votes.
Turn out was 30.22%
Read the Ham & High for Cllr Gale and the candidates thoughts on the night.