The talk show host announced the news at the top of her show on Monday, stepping out in a black dress that showed off a lot of leg in honor of her mom.
"In the name of my mother, because my mother, she's like, 'You got 'em, they're long, you show 'em,'" said Williams. "She also talked me into picking out this outfit today. She's also died."
Williams said her mother had actually died "many, many, many, many weeks ago," but "passed away beautifully and peacefully and surrounded by love." She added her mom "didn't suffer not one bit," joking it was good she didn't because of "that squawking voice."
"Our relationship was like that of 2 teenage girls, 17, if I could pick an age," she continued. "My relationship with my mother, right up to the end, was so girly, and so ridiculously giggly."
She said the two would always do their nails together while watching trashy TV and would sometimes be on the phone with each other for so long, they'd "literally fall asleep" only to have one of them wake the other up.
"Gone is the best mother, the best girlfriend that a girl could ever have," she continued. "I feel fortunate that I had her for my life all of my 56 years. People in my age bracket, many of you, throughout the years, it's like you're really lucky to have your mom and dad together in a happy marriage."
"For me, my thing is, I gotta be strong for my son. This is his first death, he does not know death, he's 20," added Williams. "So, if I get on the phone and I'm breaking down and hooping and hollering and rolling around on the ground - I'm Baptist, so you know we do all that -- but if I do all that to him, then he's going to be worried about me, then he's going to be worried about his grandfather, and his aunt ... he's gonna be worried about everyone in the family and question maybe he's not acting right."
She said she and her siblings are all looking to their father -- who is "doing well" -- for strength, while their children are looking to them for the same.
"My son is looking to me and so I'm giving him the strength that he needs. It's not like I don't cry ... I just don't cry in front of you regarding this particular thing," she added, before praising herself for not crying at all during the segment.
"Wow, I made it. Not even a misty blue. Not even nothing," she added, before joking, "What kind of robot have I become?"
"I'll deal with that in therapy," she added, "where my mother attends and secretly sits on the other end of the couch. She's everywhere."
Watch a 2010 appearance Shirley made on her daughter's show below: