The vote was called in Westholme Ward after Independent councillor Anthony Brand stepped down to concentrate on his district council role and was held on Thursday, August 4.
Alison Snookes gained the vacant seat for the Lincolnshire Independents with 220 votes, beating Conservative Nadim Aziz (133 votes) and Independent Paul Coyne (35 votes).
The turnout for the by-election was 20.2 per cent.
Alison Snookes was mayoress when her husband, Coun Adrian Snookes, was Mayor in 2019. An Independent, serving on the council for seven years, he said he intended to join her on the Lincolnshire Independent group next year if re-elected.