House Speaker Nancy Pelosi, a Democrat, delivered the oath of office Thursday morning to Tenney in the House chamber in a brief, socially distanced ceremony.
Tenney, 59, built a substantial Election Day lead based on votes cast at polling stations, but her advantage shrank to nearly nothing as a record number of mail-in ballots, mostly cast by Democrats, were counted.
Important notes related to challenged ballots were lost because they had been written on sticky notes that lost their adhesiveness and fell off.
In the end, a judge ruled that Tenney won by 109 votes out of nearly 319,000 votes cast, or a margin of just 0.034%.
The victory gives Tenney revenge for her 2018 loss, but she returns to a Washington in which Republicans have lost the White House, the House and the Senate.