The Australian received an offer from Murray while she was preparing for the French Open, the tournament she won at the start of June to land her maiden grand slam singles title.
But 23-year-old Barty said a prior agreement to play women's doubles with Victoria Azarenka at Wimbledon meant she had to decline Murray's proposal.
She feared her schedule becoming overloaded at a tournament where she is targeting Major singles success, and predicted "a million girls" would be queueing up to play with Murray.
"Andy text me before the French [Open] had started and just asked if I was keen to play," Barty said.
"I'd already committed to playing doubles with Vika so I think for me playing three events in a tournament is a bit too much, but it was definitely the hardest decision I've ever had to make to say no."
Murray will not be searching high and low for a partner after a succession of WTA players suggested they would love to play mixed with him at Wimbledon.
Former tour stars Billie Jean King and Pam Shriver also jokingly said they would fancy pairing up with the Scot, who has won the Wimbledon men's singles title twice.
Barty understands why so many would flock to line up alongside the former world No.1.
In an interview posted on Twitter by the Lawn Tennis Association, Barty said: "When a champion like Andy asks you to play I tried to justify it in my mind in so many ways, but it just couldn't happen this time.
"But I'm sure he's got a million girls lined up and he can take his pick."