Alice Thompson had wanted to work shorter hours to pick her daughter up from nursery, but ended up resigning.
The former estate agent spent tens of thousands of pounds pursuing the case against her former employer.
She was awarded nearly £200,000 by an employment tribunal, and this week told the BBC it was "a long, exhausting journey".
The tribunal ruled she had suffered indirect sex discrimination when the Manors estate agency refused to consider her request.
Ms Thompson was a sales manager at the firm in central London before she became pregnant in 2018.
When she wanted to return to work after maternity leave, she asked her employer if she could work shorter hours, a four-day week, and leave at 5pm, rather than the normal end-of-day at 6pm, to pick her daughter up from nursery.
But her manager said they couldn't afford for her to work part-time.
"I made a request for flexible working that wasn't seriously considered," said Ms Thompson.
"I proposed what would have worked for me. If that didn't work for the company, I would have been more than happy to hear a counter offer, what might work for them.
"If they needed me for the full hours - maybe 8am to 5pm instead of 9am to 6pm - that's something I could have worked around.
"But it was shut down, every avenue, not listened to, not considered. And I was left with no other option but to resign.
"How are mums meant to have careers and families? It's 2021 - not 1971."
She said she was motivated to pursue the legal challenge in order to prompt change.
"I've got a daughter and I didn't want her to experience the same treatment in 20, 30 years' time, when she's in the workplace."
She said her effort was worth it to "stand up for what is right".
The tribunal found that the firm's failure to consider more-flexible working put Ms Thompson at a disadvantage, and upheld her claim.
She was awarded almost £185,000 for loss of earnings, loss of pension contributions, injury to feelings and interest.
The tribunal said: "Losing a job unexpectedly is always a cause of unhappiness, shock, and sometimes anger, as shown by the way many employees react to redundancy, even when there has been proper consultation, and even when it is never suggested their performance was not good enough.
"Here the claimant resented that flexible working appeared not to be considered properly - as in our finding it was not - and felt that this was an injustice because of her sex, which it was."
However several other claims Ms Thompson made regarding her treatment, including discrimination because of pregnancy and maternity, and harassment, were denied by the tribunal.