Thornton won the by-election with 13,342 votes, beating Diane James for UKIP with 11,571 votes, Maria Hutchings for the Conservatives with 10,559 votes, and John O’Farrell for Labour with 4,088.
The by-election was triggered by the resignation of former secretary of state for energy and climate change Chris Huhne after he admitted to perverting the course of justice in relation to a charge of speeding in 2003.
Thornton has been a parish and borough councillor since 2007 and most recently worked as a mortgage broker for Southampton-based firm Simply Finance.
Simply Finance director Peter Williams says: “Mike has worked in the financial services industry for a number of years and had been with us for around 18 months. He is a very personable and well- liked person and one who will do really well in this new position.”