The 55-year-old actor, who plays consultant Harry Harper in the BBC hospital drama, hit John Lilley in a narrow lane near Taunton, Somerset, in 2005.
But instead of stopping to help, he made the split-second decision to continue on to Casualty rehearsals in Bristol.
Half an hour later, his conscience got the better of him and he called an ambulance to the scene.
MacCorkindale then drove to a police station to report the accident.
‘I had a complete aberration,’ he says now. ‘I made a mistake. I was horrified by what I did then and I remain horrified now. I just clipped this chap and over he went.
‘It didn’t look serious. Nobody was killed or apparently badly hurt but that wasn’t the point. It was so completely unlike me.’
In fact, Mr Lilley had suffered a fractured skull, broken elbow and cuts and bruises. He also lost his sense of smell and taste.
The accident happened while the actor was making his way through the lanes of Somerset from the home he shares with his wife Susan George on the edge of Exmoor.
He came upon the bicycle round a tight bend but, instead of giving it a wide berth, turned inwards and hit the rider with a wing mirror.
As MacCorkindale now admits, he was fully aware of what had happened: he pulled up and looked out of his window as the victim tried to get back on his feet. And then he simply drove off.
In July 2005, two months after the collision, a remorseful Simon was banned from driving for two years and fined £500 for leaving the scene of an accident. Magistrates also ordered him to pay £5,000 compensation to Mr Lilley.
MacCorkindale says he is still plagued by guilt over the incident.
‘I think I got off lightly,’ he says. ‘Friends said it could happen to anybody. They told me not to beat myself up over it.
‘But I feel I let myself down, let the show down and, come to that, let the nation down. I set myself high standards and if I fall short, I am not pleased.
‘Perhaps I was working too hard. I don’t know. But that was no excuse.
‘I do know it was one of the toughest days I have had in my life. I was in a terrible state.
‘The following day I pulled myself together and rang the chap I had knocked over.
‘I spoke to his wife and she said, ‘How could you have done this?’ and I told her, ‘I don’t know. Panic took over. But if you knew me you’d know I don’t do stuff like this. The whole experience left me totally shattered and caused me many a restless night. And so it should have.
‘It gave me more of an empathy, understanding people who are in trouble.
‘This was stuff you could draw on, learn from. It certainly convinced me that there is a way to behave in certain situations. It was so much like Casualty.’
The actor is currently on tour in a revival of the Agatha Christie thriller The Unexpected Guest.
It is the first time he has taken to the stage in 21 years and he admits to being ‘fairly nervous’ until the curtain goes up each night.
After three decades in the acting business – including an ill-fated move to Hollywood in the Eighties – he remains only too aware of its unexpected highs and lows.
Although he returns to Casualty this summer and is contracted to remain until the end of the year, he jokes that anything could happen.
‘They will probably turn Harry into a serial killer, make him go bonkers and throw himself off a cliff,; he says with a laugh. ‘That’s the way things go.’
The Unexpected Guest by Agatha Christie is touring the UK until June