The will left Rooney's estate to a stepson and nothing to his wife, Janice, or other children.
Attorney Eugene Belous told CNN he would be in Los Angeles County Probate Court on Monday morning to argue that the will contains "a blatant misstatement" about agreements between her, the actor and his conservator.
"There is NO provision in either of the two settlement agreements ... that terminates or in any way effects (her) rights as surviving spouse," Belous said.
A court filing says Jan Rooney signed an agreement waiving all claims to her husband's estate after the couple separated in June 2012 after 34 years of marriage.
Mickey Rooney's court-appointed conservator, who is also named as executor in the will, told CNN on Friday that the challenge of the will "is totally without merit."
Since there is not much money to fight over, attorney Michael Augustine said Rooney's widow, who was separated from her husband the last two years of his life, was seeking public attention with the challenge. "She cannot resist her additional five minutes of fame," Augustine said.
Although Rooney disinherited his children, his wife and all but one of her children in a will he signed just weeks before his death, court papers suggest there is not much in his estate to fight over. His personal property is valued at just $18,000 despite an unmatched 90-year film career.
The will signed by Rooney on March 11, 2014, left the entire estate to stepson Mark Rooney, one of Jan Rooney's sons, who was the actor's caretaker the last two years of his life. He died on April 7.
Rooney "intentionally omitted" and disinherited his eight surviving biological children and two other stepchildren from his last marriage, the will said.
Rooney had no negative feelings toward his surviving children, but they were all financially better off than he was, Augustine said, adding that Rooney believed that what little he had to leave should go to Mark Rooney and his wife, because they had been taking good care of him in his final two years.
A hearing is scheduled for Monday to start the process of probating the actor's will.
An attorney for Mickey Rooney's children is also contesting the will, Belous said.
Augustine said there was no question that Rooney was of sound mind when he signed the will last March. The actor had just finished filming his "Night at the Museum" scenes with Dick Van Dyke, in which he had no trouble learning his lines, he said.
An independent lawyer appointed by a judge confirmed that he found Rooney to be "perfectly competent," Augustine said.