Joe Cardle, Kallum Higginbotham, Nicky Clark and Andy Ryan were all on target as the East End Park men swept away a Sons side whose misery was compounded by a second-half red card for Andy Dowie.
Johnston said: “We’re starting games really fast. We’re making a lot of chances and it is great to be scoring so many goals.
“We’ve got a lot of pace in the team and you can see the different striking options that we have.
“We’ve improved but there is a long way to go. We’re going to have to work even harder if we want to stay up near the top of the league.
“I’m delighted with the way we’ve started as I think that was our downfall last season.”
The visitors got off to the perfect start when they opened the scoring after just five minutes.
A Declan McManus shot was cleared off the line by Dowie, who then blocked from Callum Smith, but couldn’t prevent Cardle, pictured, from notching the opener.
The Pars deservedly doubled their advantage thanks to a fantastic counter-attacking move, which culminated with Higginbotham firing the ball home.
Dumbarton had offered nothing, with midfielder Tom Walsh’s effort from distance which flew over the bar their best opening.
The Pars had a chance to make it 3-0 just before the interval but Dean Shiels headed wide from Ryan Williamson’s deep cross.
Dumbarton looked to have perked up after the break and came close to reducing the deficit when Craig Barr’s header from a Walsh corner struck the bar.
However, the game was up five minutes later when Dowie chalked up his second booking and was red-carded for a trip on Pars sub David Hopkirk.
Dunfermline netted a third when Hopkirk’s cross was headed home by Clark, whose cheeky back-heel then rolled nicely for Ryan to complete the scoring in stoppage time and send the visiting fans home happy.
Dumbarton boss Stevie Aitken said: “You can’t give a team of Dunfermline’s class and ability a two-goal start and that’s what we did.
“The second half was always going to be difficult and there was plenty of effort but we were well beaten.”