The 20-year-old, who plays Whitney Dean on the Walford soap, was punished after being previously warned about her persistent lateness to arrive on set, according to the Daily Star Sunday.
She was reportedly told of her suspension personally by EastEnders executive producer Lorraine Newman, who was said to have "read the riot act" to McGarty after the actress's behaviour irritated cast members and crew.
A BBC source revealed to the paper: "Shona failed to turn up on time on more than one occasion and was really laid back about why she was late. People are understanding when it's once or even twice but her lateness was becoming a huge problem.
"It regularly held up the filming of vital scenes and angered fellow cast and crew members who were forced to work late because of her. Like all soaps, everyone works long, hard days so when things are prolonged because of one person it doesn't go down well on set.
"There were a lot of unhappy people so Lorraine had no choice but to take action. Shona was read the riot act and apologised for her behaviour. Obviously, she's not happy about what's happened and was quite upset about it all afterwards."
An EastEnders spokesperson also confirmed: "Shona has been suspended for repeatedly being late."
McGarty was previously reported to have got herself into trouble after throwing yoghurt around in a branch of Asda in March, along with boyfriend and co-star Matt Lapinskas (Anthony Moon).