Peterborough saw Darren Ferguson depart the club by mutual consent following a 3-0 defeat to MK Dons in their previous League One outing, but caretaker manager Robertson made an instant impact, guiding his side to a 2-0 win over play-off hopefuls Bradford City.
Gabriel Zakuani and a late Michael Bostwick penalty did the damage, moving United up to 10th and within four points of the play-off places.
Johnson's first test at the Barnsley helm took him to Gillingham, who were unbeaten in eight matches, and the visitors were up to the task, securing all three points through George Waring's 56th-minute finish.
At the top of the table, it proved to be a good day for pacesetters Bristol City, who beat Rochdale 1-0 to open up an eight-point lead at the summit.
Two of the chasing pack dropped points, as MK Dons lost 2-1 at managerless Coventry City, while Swindon Town were held to a goalless draw at Crewe Alexandra, with both Alan Tate and Jake Stephens sent off.
Preston North End made the most of Swindon's slip-up and moved up to third thanks to an emphatic 4-0 win at Oldham Athletic, their fifth successive league triumph.
An 81st-minute Mark McNulty penalty salvaged a 1-1 draw for Sheffield United at Crawley Town, while Doncaster Rovers consolidated their position in the play-off places with a 2-0 defeat of lowly Colchester United.
Towards the bottom of the table, Leyton Orient gave their survival hopes a boost with a 2-0 victory at Walsall and moved to within a point of safety, though Yeovil Town remain last after drawing 1-1 at Scunthorpe United.
There were also wins for Port Vale and Chesterfield, defeating Notts County and Fleetwood Town 1-0 and 3-0 respectively.
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