Ahmed's 105 was the cornerstone of Pakistan's 251 all out -- a seemingly below par total but one that looked well out of reach as they slumped to two for three following captain Azhar Ali's decision to bat first.
Imad Wasim's 63 not out was also his career-best score at this level and the left-hander helped Ahmed add 77 for the sixth wicket.
Pacemen Chris Woakes and Mark Wood took three wickets apiece as England looked to go 2-0 up in this five-match series following their 44-run rain-affected win at Southampton on Wednesday.
There was no hot-spot mark but Australian third umpire Simon Fry decide the spike on real time snicko was large enough to give Aslam out.
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But there was no doubt at all when Wood dismissed Sharjeel Khan for nought with an express delivery that cut sharply off the seam to knock over the left-hander's off stump.
Azhar also fell for a duck when caught behind off Woakes.
Babar Azam and Ahmed kept England at bay, with Babar (30) hitting several stylish boundaries before he played on to Plunkett.
But Ahmed, as he'd often done during the preceding 2-2 drawn Test series, frustrated England with a mixture of orthodox and improvised shots.
Shoaib Malik (28) struck a straight six against off-spinner Moeen Ali in a fifth-wicket partnership of 59 before Wood's return saw him caught behind.
Ahmed, whose lone previous ODI hundred was against Ireland at last year's World Cup, completed a 124-ball century when he leg-glanced Plunkett for his sixth four.
He then successfully reviewed Plunkett's lbw appeal before he slog-swept Ali to Alex Hales in the deep.
Wasim struck Plunkett for three boundaries in as many balls -- an uppercut six followed by a straight drive and a cover drive -- to go to a 57-ball fifty.