Ms Sawant, an economics college professor, campaigned on taxing the wealthy to fund city needs
Seattle voters have elected a socialist to city council for the first time in modern history.
Kshama Sawant's lead grew to 1,640 votes on Friday, prompting 16-year incumbent Richard Conlin to concede.
Conlin was backed by the city's political establishment, but Ms Sawant's grassroots and message of inequality resonated with the city. On election night, Ms Sawant was losing by four percentage points, but ballots counted after election day skewed heavily toward her.
Ms Sawant, an economics college professor, campaigned on taxing the wealthy to fund city needs and backs a $15 minimum wage.
The last socialist candidate to make it into the general election was in 1991 and was defeated easily.