Mr Muspratt (Lab) made no secret of his animosity towards the Conservative-run county council.
He said: 'People have felt the county council was not listening to concerns in Watford. It often puts party concerns before those of the town.'
Ms Thornhill said this was a major factor in her victory, claiming residents had had enough of the borough council's adversarial attitude towards the county.
A source at Hertfordshire CC said officers and members look forward to better relations with the borough council.
Leader Robert Ellis (Con) added triumphantly: 'This result neatly demonstrates the truth that nothing in politics can ever be taken for granted.'
A spokesman for the council said it was difficult to judge how much of the animosity was personal and how much was political.
Turnout was higher than expected at more than 37% and Ms Thornhill beat Mr Muspratt by more than 8,000 votes in the surprise victory.