He then asked a "staff member" to unlock the door and walked in on her, telling her she should have either left the door unlocked or pumped within the public areas of the terminal.
McBain says insult to injury was added after she wrote a letter to the airport's customer service department complaining about the incident.
The response, which was badly composed, said: "While it was uncomfortable it was not entirely his fault for entering after warning."
It continued: "With that said, every passenger should be treated with courtesy and respect, meaning we expect our staff to be professional, helpful and polite at all times."
Speaking to MailOnline, McBain said: "The hundreds of breastfeeding mothers who use Heathrow daily deserve a clean, private room in which to pump or nurse without being disturbed, and a toilet or a baby changing room doesn't cut it.
"Just as no one would be spoken to while sitting on the toilet, it's respectful to wait until a woman's breasts are covered before telling her where to pump.
"But the bigger issue is that when I raised my problems with Heathrow their response showed no willingness to address my specific concerns for breastfeeding women."
The mother, who says she hadn't planned to tweet about the experience but changed her mind after reading Heathrow's response also told The Huffington Post: "From my perspective what's really important is that this isn't about one male attendant's awkward reaction - he was probably expecting an empty cubicle and we sometimes say weird things when caught off guard."
Many on Twitter shared their frustration over the experience. @EmmaElinor tweeted: "I'm so sorry and angry to hear what happen and to read the reply. It's so infuriating that so many spaces aren't fitted for the needs of people with small children, whether parents are travelling with them or not. Tiny changes can make an enormous difference."
@O_edipus added: "It's quite telling that an official response from one of the first public bodies one comes comes into contact on entering the UK contains spelling mistakes such as 'would of'."
A spokesperson for Heathrow Airport told MailOnline: "The response Ms. McBain received following her complaint did not reflect the level of customer service we expect.
"We apologise the initial response was handled in the manner it was and regret the experience she had at our airport.
"Following this incident, airport and customer service colleagues will be spoken to about service delivery and how it needs to be tailored to individual customer needs."