The woman, whose name is unknown, was travelling 5,000 miles to visit her family for company, as her husband had been deployed.
"My biggest apprehension was making such a long trip. With a toddler. By myself," she wrote on the Love What Matters Facebook page.
"I had such anxiety about her airborne tantrums. Once before, I had a very rude man tell me to: 'Shut that child up before I do', when my very tired infant would not stop crying.
"However, this time was very different."
The mother said she was working "diligently" to try to prevent an outburst from her daughter.
"But it escalated," she wrote. "Hysterical crying. My heart raced as I tried to ignore any irritated stares and eye rolls and focused on soothing her.
"But then the man sitting next to us did something I didn't expect. He told me with a smile, 'You're doing great, mum'."
The mother said the stranger suggested she switch seats with her daughter so he could sit next to her. She agreed.
"He pulled out his iPad to show her pictures of his grandchildren," the mother continued.
"He opened a game and showed her how to play it. He talked with me about his life and asked me about mine.
"He helped occupy my daughter and soothed her when she fussed. He was so patient and loving.
"Before I knew it, the six-hour flight was coming to an end.
"We had survived. And the man helped me with my bags as I carried her off the plane. He walked us to the baggage claim where my parents greeted us.
"Then he said goodbye and quickly disappeared. I explained to my mum with tears in my eyes what this man had done for us."
The mother said she was grateful that the stranger didn't judge her, grow irritated or make any rude comments.
"He gracefully sacrificed his time and energy to help a distressed mother and child," she wrote.
"He was understanding. He made our flight, not just bearable, but pleasant.
"It was the kindness of a stranger that got us through that day and I will always be thankful for him."
The mother's story has been shared more than 500 times in four days since being shared on Facebook.
"I'm in tears, what a wonderful man," one person commented.
Another wrote: "I love this story! As a frequent flyer with many small children I have also born the brunt of rude passenger comments.
"I pray that one day I will remember to be like your daughter's seat mate."