She had defeated her BJP rival by 37,000 votes in the by-polls held on April 11.
With this the Harish Rawat-led Uttarakhand government has secured majority in the 70-member Assembly House with 36 seats.
The oath was administered by Assembly Speaker Govind Singh Kunjwal in the presence of the chief minister and party workers.
Mamta said people have given her a "huge mandate" not just to be an MLA but to "serve as a minister".
Surendra had died due to cancer in February.
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