Simran Cheema polled almost 65 per cent of the 1,090 votes cast on Thursdaybut was unable to sign the declaration of acceptance of office of councillor after it emerged she was ineligible to stand at the time of her nomination.
She apologised to Pleck residents and said she had been unaware her part-time job with Walsall Libraries prevented her from standing.
Her victory over the only other candidate standing – Conservative Mohammed Saghir – means the seat will remain empty until May when it is up for election again.
She polled 698 votes with Mr Saghir getting 382, and 10 papers were spoiled. The turnout was a lowly 10.66 per cent.
Earlier this month, the Tory council boss Mike Bird called the situation a ‘farce’, adding it has cost an estimated £20,000 to host the by-election.
It is understood neither Walsall's Labour or Conservative leadership teams wanted the ballot to be held as the seat is going to be contested again in May anyway.
After the result, Miss Cheema said: “I would like to thank the people who voted for the Labour Party to return.
“I’d just also like to offer them a huge apology on my behalf that unfortunately they won’t be able to have a representative this time. But hopefully in May they will give their support to us again.
She added: “I have been overwhelmed with the support I have received from all over Walsall following the circumstances regarding my candidacy.
“Following my nomination, I was intent on tackling the most pressing issues in the ward and borough and standing up and representing.
“Unfortunately, at the time of my candidacy, I wasn’t aware my part-time employment at Walsall Libraries would prevent me from standing for the council.
“For that, I can only apologise to residents for not being aware of the complexities of electoral law.
“This setback has only made me determined and not deterred to carry on working in the community with the Pleck Labour team to make Walsall a better place to live, work and study in.”
Walsall Labour group leader Aftab Nawaz said they had ceased campaigning when the issue came to light and added he was pleased people in Pleck still backed the party.
Mr Saghir said: “It is a very safe Labour seat. We worked very hard and it was a good campaign.
“Normally we only get around 600 votes when the election is in May so we got nearly 400 this time.
“There is a winner and a loser. It’s a big anti-climax really. I will go again in May but not in Pleck.”