United's pursuit of a top-six finish suffered a hiccup early on, though, as Jon Taylor headed the hosts into a one-goal lead after just five minutes.
The advantage proved short-lived, as the 21-year-old bagged his first of the afternoon seven minutes later to restore parity.
The visitors took the lead on 21 minutes as Washington coolly slotted beyond goalkeeper Chris Weale following an eye-of-the-needle pass from midfield.
Isgrove then vindicated Ferguson's decision to hand him a starting berth by trebling the Posh lead shortly before the half-hour mark.
Tom Bradshaw threw Town a lifeline in the second half, reducing the arrears on the stroke of the interval as Ferguson's side boast a one-goal half-time lead.