First-team coach Matt Hamshaw has also extended his time at the club, with the trio having had ten months remaining on their current deals.
Warne is in the fourth year of his official managerial reign with the Millers, having been appointed full-time in April 2017 following a brief spell as caretaker.
Assistant and former team-mate Barker arrived as his number two on the final day of the 2016-17 campaign.
During their tenure, the club have recorded two promotions in 2017-18 and 2019-20 and transformed the club's profile and standing on the pitch.
On the trio signing fresh terms, chairman Tony Stewart said: "I am ever so pleased to get them over the line because they are proven, tried and tested.
"They started with us three seasons ago now. There is good team work between the three of them, they work well together and they have proven themselves.
"Paul Warne and his staff are young, vibrant, intelligent and part of the progress we have made. There is a real good feel factor around them.
"The communication is superb, they want what we want and they are going nowhere.
"It’s a win-win, what’s nice to tell the fans is that they wanted longevity at Rotherham.
"They have shown what they can do and I still think they can achieve more. They are still young, but learning very quick and it shows what they can produce over the last few years, it is incredible."