"What I would say to all of Mr. Cosby's fans and some of the folks on the other side of this, we have a wonderful criminal justice system in this country," McMonagle said on ABC's "Good Morning America."
"Trust it, believe in it, and I'm confident that if this case is retried, he'll be acquitted," McMonagle said.
Asked what he would say to people who have heard about other accusers against Cosby, McMonagle said, "None of these accusations have ever been vetted. None of them have ever been put to the test."
"None of them have ever been cross-examined, and the one time that it has happened, fair-minded people couldn't come to a conclusion," he said in reference to the current case in Pennsylvania, which ended Saturday in a mistrial. "Presume innocence and trust the system."
After six days and 52 hours of deliberation, the seven men and five women selected to serve on the jury were unable to render a unanimous verdict on any of Cosby's three charges of felony aggravated indecent assault. Cosby, 79, pleaded not guilty to the charges and denied any wrongdoing in regard to other accusations.
Though he never took the stand during the trial, excerpts from his 2005 to 2006 depositions in a civil suit by his accuser, Andrea Constand, were read aloud.
For the prosecution, Constand took the stand, as did her mother and another accuser, Kelly Johnson.
The case stemmed from an alleged 2004 encounter in which, Constand said, Cosby drugged and assaulted while her she was an employee at Temple University, his alma mater. He said everything that happened was consensual.
According to portions of his deposition, he admitted to giving her Benadryl to "relax" her.
Over the weekend, his wife, Camille Cosby, spoke out, taking on the district attorney in a statement, calling him "heinously and explosively ambitious." She also said the judge was "overtly and arrogantly collaborating with the [district attorney]."
McMonagle said Camille Cosby’s words will not have any effect on the case if there is a retrial.
"I know that we were personally vilified during the trial, and I think when you're looking at Mrs. Cosby, you're looking at a wife who believes that the system is unfairly treating her husband and victimizing her husband," he said.
Kevin R. Steele, the district attorney for Montgomery County in Pennsylvania, has said he will seek a new trial against Bill Cosby.
One of the judges on the case said he would like a retrial to begin within 120 days. The prosecution said that legally it has 365 days to retry Cosby.
The prosecution also said it will revisit its petition to have 13 of the more than 50 other Cosby accusers testify in the retrial.
Constand's lawyers released a statement on Saturday after the hung jury result, thanking the district attorney's office and "the many police officers and detectives who worked on this case," as well as the "jury for their tireless efforts and ... their sacrifice."
"From the moment she revealed what had happened to her, Andrea sought to have this matter addressed in the criminal justice system," the statement added.
"We're only here because of those unproven accusations, and I believe and I know in my heart that the judge will not reverse his decision, will give us a fair trial as he just did," McMonagle said.
He added that if Cosby asks him to defend him in a retrial, he would "answer the call."
"Jury deliberations were 52 hours, so we're all trying to mend and heal, and we'll make some important decisions in the days to come," McMonagle said.
ABC's Chris Francescani and Linsey Davis contributed to this report.