Freeman starred as Dr John Watson in the hit TV show from 2010, alongside Benedict Cumberbatch in the title role.
Abbington – who was with Freeman for 16 years and has two children with him – joined the cast as Mary Watson in 2014.
However, she has now suggested that “nepotism” was at play.
Speaking on the Full Disclosure podcast, Abbington said: “I was with Martin at the time, there was a lot of nepotism being banded around.
“It's true, it was that.
"Both Mark and Steven (the show’s creators and writers Mark Gatiss and Steven Moffat) said, look, we were trying to work out who would be the best person to play Mary, John Watson's wife, and we thought it would be the person who was going out with him.”
“So it makes sense,” said the 48-year-old.
“And we had worked together on many things. I think we had worked together four or five times before that."
Abbington said she and Freeman enjoyed working together and it “seemed like a natural thing”.
“I was over the moon because I loved that show,” she added.
Abbington and Freeman were a couple from 2000 to 2016, and they have a son and a daughter together.
The actor is now in a relationship with former America’s Got Talent and Britain's Got Talent star Jonathan Goodwin.
The escapologist nearly died after falling 30 feet between two burning cars and missing a safety airbag below when a stunt rehearsal went wrong, and now uses a wheelchair.
Watch: Jonathan Goodwin told Amanda Abbington she could leave him after accident