Ben Diggle, 28, Halifax, said he noticed three-year-old Imogen's eyesight wasn't as developed as her twin brother George's so took her straight to the doctors.
Imogen was diagnosed with a brain tumour resting on her optic nerve in February, after optician specialists rushed Imogen in for a two-hour MRI scan and is now permanently blind in her left eye.
Mr Diggle said, according to the Mail Online: "We were absolutely devastated when they told us. You think beforehand that you can be prepared for something bad but it's a shock when it hits you."
Last October when he took her to her GP, the doctor referred the dad to an optician, who said Imogen was "too young" for the test to show anything significant.
After the initial eye test, Mr Diggle was told to return in six months but when things weren't improving, he demanded a hospital appointment, which soon uncovered the three cubic centimetre tumour.
Imogen is now having intense chemotherapy, but surgeons are unable to operate on the tumour as she could risk losing her sight in both eyes.
Mr Diggle, an insurance claims handler, said he couldn't believe that someone so small has something that big in her head.
He said he had to make a little version of it, around the same size, with a ball of paper so he could try and understand what was happening to his daughter.
The chemotherapy will not be able to regain her sight, but will reduce the size of the tumour on her optic nerve and optic chiasm.
Imogen is having another MRI scan in September to see if the chemotherapy has been successful in shrinking the tumour.
Mr Diggle was told that the tumour will not get worse as it is a grade one - the least aggressive kind, and said Imogen seems happy and not too deeply affected by it.
He said, according to the Mail: "I'm confused and angry that they couldn't have looked at her sooner.
"A physiotherapist had to tell us that a hospital could do eye tests for her and I'm grateful that she did because we might not have done anything about it for another six months."
The father said aside from her illness, she is just a "normal kid" who loves Frozen, playing in the park and always asking for a McDonalds.
The family is now waiting until September to see if the chemotherapy has been effective.