He says they were driving near a toll booth when a member of the press overtook them to get a clear shot at taking their picture.
Mark says he swerved the vehicle to avoid the photographers but hit a concrete barrier, causing Samantha to hit the car's windshield before falling to the floor.
Samantha Markle suffered the injuries on Wednesday (16May18) during a confrontation with the paparazzi in Florida.
Meghan's sister, who has MS, reportedly jerked forward into the windshield before falling to the floor and Mark says he saw her foot twisted.
Samantha has been publicly vocal about her alleged experiences with her half-sister Meghan, since the announcement of the former Suits actress' engagement to Prince Harry.More news: Mario Gotze Misses Out On Germany's Provisional Squad
Samantha, who revealed she had bought a wedding gift despite not being invited, said her relationship with Meghan was "strained". She insisted it was for the benefit of the royal family and not a cheap grab for cash.
The Sun also reported that Samantha Markle had "slammed claims" that Kensington Palace was assisting their father financially during his hospital stay.
'There is something in this country called freedom of speech, ' she said. There's no bigger media vulture with this wedding than you, is there?
"She doesn't have a copyright on that and she's not going to tell me that I can't speak about my own life or my father's where it's a matter of public self-defense because the media is disparaging us".
Thomas had three stents placed in his blood vessels after suffering a heart attack.
Meanwhile, Harry and Meghan's nuptials will take place this weekend, on Saturday, May 19, at St George's Chapel in the grounds of Windsor Castle, where the prince was christened.