Detectives in Greenwich, South East London, arrested the 53-year-old in the Deptford area following the shocking incident yesterday lunchtime.
Parents in the area had been warned to beware after concern spread that more babies could be targeted.
The arrest came as the the baby's mum spoke for the first time of her terrifying ordeal. Samantha Cherry, 18, told The Sun she had gone with her 11-month-old baby Jonjo and mum Mandy, 47.
Samantha said they had turned their backs for a moment when a stranger placed a baby bottle containing a yellowy liquid into the pushchair.
Mandy said she turned round to see the toddler choking and frothing at the mouth and picked him up thinking he could have been choking on a chip.
But Samantha realised the bottle was not Jonjo's. She said she was then told by some youngsters a woman had walked past the pushchair and put the bottle in the buggy.
Staff raised the alarm and police and paramedics arrived at the McDonald's in Eltham, south-east London. Jonjo was taken to hospital with his terrified mum as police launched a search for the suspect.
Samantha, who works as a Sainsbury's assistant, told the newspaper: "After our meals I went up to buy an ice cream and when I came back Jonjo was sitting on my mum's lap.
"He had been sick and had gone all limp and pale. Then I saw the bottle with this yellowy liquid in it. Mum said to try it and before I'd even sipped it I could smell the bleach coming off of it.
I was horrified. I couldn't believe someone had tried to poison my baby.
"It was just so frightening. It was the worst day of my life."
Jonjo was discharged at 8.30pm last night and is now back home with his family but he has refused to touch much food and his mouth and throat are still sore.
Swallowing just a small amount of household bleach or sodium hypochlorite solution can be fatal, particularly for children or babies. The corrosive qualities of bleach can also cause severe and permanent damage to the throat and stomach.