The Tykes were 1-0 winners at Gillingham.
Johnson left Oldham to take over at Barnsley and saw a George Waring goal take all three points for the visitors.
Doncaster Rovers climbed back into the play-off places with a 2-0 home win over Colchester thanks to a Enda Stevens goals and a Nathan Tyson penalty.
The defeat saw the U’s slip back into the relegation zone, while Rovers are in sixth spot.
Sheffield United slipped up in their pursuit of automatic promotion, when they could only draw 1-1 at Crawley Town.
Former Sheffield Wednesday defender Richard Wood netted for Crawley, but United snatched a point with a Marc McNulty penalty with nine minutes remaining.
A Gaby Zakuani goal and a Michael Bostwick penalty saw Dave Robertson make a winning start as Peterborough caretaker manager as top-seven rivals Bradford were defeated 2-0 at the ABAX Stadium.
In League Two, York City were held 0-0 by Exeter and stay out of the bottom two on goal difference.
FC Halifax Town picked up a 0-0 draw at Kidderminster in the Conference.
And non-league North Ferriby United are on their way to Wembley in the FA Trophy.
United drew 1-1 at home to Bath, 3-3 on aggregrate, but won 4-2 on penalties in their two-legged semi-final.