Hidalgo, 54, won with 91 out of 163 votes.
Discontent with President François Hollande's government cost the Socialists millions of votes throughout the country and they lost control of several town councils.
Kosciusko-Moriset was ahead of Hidalgo in the first round of voting but lost when Greens and Communists backed the Socialist candidate in the second, gving her 53 per cent of the vote.
The new mayor, who was deputy to fellow-Socialist Bertrand-Delanoë for 13 years, is described as honest, studious and modest by her comrades and has encourage fairness and solidarity in the capital.
Like new Prime Minister Manuel Valls, Hidalgo is of Spanish origin.
She was born in San Fernando in Andalusia under the dictatorship of General Francisco Franco in 1959.
Her parents came to France in 1961, settling in the central city of Lyon, and she became a French citizen at the age of 14.
She moved to Paris in 1984 and has a law degree.
Hidalgo joined the Socialist party in 1994 and worked with Lille mayor Martine Aubry when she was labour minister in 1997.
She has three children, one with her current husband, Jean-Marc Germain, and two from a former relationship.