Dominic Colella was called out on 9 March 2013 when the 85-year-old man collapsed in a queue at a London branch of the store.
A colleague told a disciplinary hearing Mr Colella had returned to the ambulance with two "full bags of shopping" after 20 minutes.
He did not attend the hearing.
Giving evidence to the Health and Care Professionals Council, fellow paramedic Yvonne Purves said the elderly man had passed out in M&S and then regained consciousness before being moved into Mr Colella's ambulance.
She told the panel she had wondered where her colleague had gone after waiting for some time in the ambulance.
"The wife was understandably anxious about what had happened to her husband and was wondering why we weren't leaving to go to hospital," she said.
"He then came back with two full bags of shopping and loaded them in to the front of the ambulance and flippantly said 'do you want to go to hospital then?'.
"Obviously, I wanted to go to hospital some time ago. I waited 20 minutes for Mr Colella while alone in the ambulance," she added.
On another occasion, Mr Colella is alleged to have got a hair cut instead of taking a head injury patient to hospital.
The panel heard Mr Colella, who faces two counts of misconduct, did not turn up at the hearing because he was working.
He has since resigned from the London Ambulance Service.
The hearing continues.
A paramedic shopped in Marks and Spencer while an elderly patient with severe blood poisoning waited in an ambulance, a conduct panel has heard.