Jonson Clarke-Harris was the scorer for Peterborough, ensuring all three points and scoring his 26th league goal of the season - but Fleetwood missed some close-range chances.
Peterborough United had the chance to take an early lead, when Clarke-Harris made space for a shot from 12 yards out that was denied by the Cod Army keeper.
Around the half-hour mark, Fleetwood began to apply some pressure; Ged Gardner and Wes Burns spurned big chances before the break. The Cod Army kept that momentum into the second half, creating the best chances including one pushed to safety and one taking a deflection to hit the bar.
Ultimately, it was an 89th-minute free-kick that claimed Posh a hard-fought 1-0 win.
Speaking after the game, Posh boss Darren Ferguson was under no illusions that his team passed an important and difficult test at Highbury, saying: "It is a big win for us, a late goal and that can often be the best time to score as there is no time for the opponent to come back at us.
"I fancied Jonno (Clarke-Harris) when the free-kick was given. It was a great strike and it is another clean sheet and they are going to be so important, they really are.
"It is another win away from home, it is a much-needed away win and I thought it was a game that we started both halves really well in.
"Towards the end of the first half we petered out a little. We just needed to up it and we would go on and win it and so it proved.
"We made the changes. Jack (Taylor) was outstanding when he came on. Siriki (Dembele) gives you that unpredictability going the other way and he got into a couple of really good positions.
"I named the same side because we were so good in the win over Accrington. I knew I had good options from the bench to change it if we needed to.
"We can now look forward to a big game on Monday against Sunderland, live on the TV at home, it is just a shame there are no fans to see it in the stadium. That game won’t define our season, there are still a lot of points to play for."
That kept Sunderland from moving into the automatic promotion places, but a Hull City win means Posh still sit in second.