The Ayrshire side suffered defeat at the hands of Aberdeen in their first match at Rugby Park but looked in much better form against the Steelmen.
Goals from Robbie Muirhead and Sammy Clingan sealed the points against 10-man Well, who had Fraser Kerr sent off late on.
Johnston said: "You couldn't get two harder games at home - Aberdeen and then Motherwell.
"You look at Motherwell's squad and it's more or less the same as last year and they brought in Ojamaa, who's a top player.
"But I thought we created a lot of chances and defended well. It was good to get a clean sheet as well.
"There were times we passed the ball well and we had to defend well too but I don't think we looked like we were going to lose goals."
Motherwell boss Stuart McCall, whose side have lost three league games in a row, said: "The second goal just summed things up for us, the second goal, with 10 men, just killed it off.
"Stephen McManus collapsed in a heap, the ball bounced around and Fraser Kerr made a challenge and they scored from the free-kick.
"Fraser Kerr doesn't have a malicious bone in his body but it was a late challenge. It looked bad, it looked like a red card."