Former chairman of Lancashire County Council Wendy Dwyer comfortably won the Peel ward by-election for Labour.
And party chiefs have accused their opponents of smearing the ex-Accrington South county councillor in the election campaign over her age.
The seat became vacant followed the resignation of Graham Jones as ward councillor, following his election as Hyndburn MP.
Mrs Dwyer, who is in her seventies, received 592 votes to defeat the challenge of Conservative Danny Cassidy, on 189 votes. The turnout was 25.7 per cent. The new councillor, of Stanley Street, retired last year after 16 years at county hall due to family commitments.
But she has now returned to represent the ward where she has lived for over 50 years.
Group leader Councillor Miles Parkinson said: "It is a very positive result for Labour locally and it has defied the Conservative literature, which was 'vote for the future not the past'. I do not think it was very appropriate to attack Wendy at all because of her age."
Mrs Dwyer said: "I'm obviously very pleased I won and I will do my best to help the people of Peel. It's been seven years since I was on the district council but I only finished on the county last year so I've still been in the loop and know what's going on."
The result returns the town hall to the same position immediately following May's local elections, with the Tories on 17, Labour 14 and four Independents.