Nicole Sedgebeer, 21, hopes to raise £1,000 to help Mark who she says "embodies the best of humanity".
The Just Giving page has already surpassed this with 437 people so far pledging £5,307.
She wrote on Facebook:
Sedgebeer had missed her train from Euston Station to Milton Keynes on 3 March when Mark came to her rescue just as she was "about to burst into tears".
He led her to an all-night cafe where she could stay in the warm and even came back at 5am to make sure she got the train back.
In a post on Facebook, Sedgebeer explained:
The post reads: “I hope this story makes people look twice when they see a homeless person.
“Last night I missed my last train home and when I finally got to Euston to wait till the morning, the station was locked. Just as I was about to burst into drunken tears I met my homeless friend Mark."