England dismissed Pakistan for 304 on the first day of the third and final test of the series at Karachi's National Stadium on Saturday, with home side skipper Babar Azam and Agha Salman putting on defiant half-centuries.
Leach grabbed 4-140 and Ahmed finished with 2-89 as Pakistan batters spoiled good starts after Babar Azam won the toss and batted on a turning National Stadium pitch.
But England also fell prey to spin, with mystery tweaker Abrar Ahmed trapping Zak Crawley without scoring before the visitors closed the day on 7-1.
Ben Duckett and Ollie Pope were at the crease with four and three respectively as England trail by 297 runs.
England lead the three-match series 2-0 after winning the first test by 74 runs in Rawalpindi and the second in Multan by 26 runs.
Skipper Azam top-scored for Pakistan with 78 before running himself out in the post-tea session, while Agha Salman made 56.
But the start of the day belonged to Ahmed -- who at 18 years and 126 days old became the youngest England player to start a test -- when he dismissed Saud Shakeel for 23 in his seventh over.
Before Ahmed, Brian Close was the youngest England debutant at 18 years and 149 days when he played against New Zealand in 1949.
Leach then came into the act, breaking a stubborn eighth wicket stand of 48 between Salman and Nauman Ali (20).
Ahmed, who was presented with his test cap in front of his Pakistan-born father Naeem by former England skipper Nasser Hussain, described the scene as "the best moment".
"It's great to be part of it. I believe in myself and try to do my best when given an opportunity," he said.
"They (England players) backed me the whole day and I couldn't ask for more. To play at such a young age is a blessing. It's the best feeling in my life."
'GOOD PROSPECT'
Pakistan's head coach Saqlain Mushtaq, also a great spinner in his days, praised the youngster.
"It was a very impressive performance considering it was his first game. He is a young chap and he has a lot of potential and looks a very good prospect for the England team."
Saqlain thought his team fell short by 60-70 runs.
"It's not a bad total, some 60-70 would have been better, but we have to play disciplined cricket to restrict them because I think the pitch will get slower by the day."
In the morning session, Pakistan lost Abdullah Shafique (eight), Shan Masood (30) and Azhar Ali (40) before Azam and Shakeel looked to have steadied the innings with a 45-run fourth wicket stand.
But Ahmed had Shakeel caught off a defensive shot at short-leg before Azam was adjudged run out by wicketkeeper Ben Foakes after a lengthy review.
Azam, who hit nine boundaries, also added 71 for the third wicket with Azhar.
Azhar -- playing his last test -- fell at the stroke of lunch when he gloved a catch to a diving Foakes off pacer Ollie Robinson.
Sensing the pitch would take spin, England started the attack with Leach, who trapped Shafique leg-before in the sixth over of the day.
Leach becomes the first spinner to open the attack in a test for England in 101 years -- Jack White did the same against Australia at Leeds in 1921.
Masood fell to the trap of short bowling as he pulled fast bowler Mark Wood straight into the hands of deep fine-leg fielder Leach.
Masood hit five boundaries while Azhar had six hits to the ropes.
Under pressure to avoid their first-ever 3-0 whitewash at home, Pakistan made four changes from the second-test line-up while England made two, bringing in Foakes and Ahmed.
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PAKISTAN: Babar Azam (capt), Abdullah Shafique, Azhar Ali, Shan Masood, Saud Shakeel, Mohammad Rizwan (wk), Agha Salman, Faheem Ashraf, Mohammad Wasim Junior, Abrar Ahmed, Nauman Ali
ENGLAND: Ben Stokes (captain), Harry Brook, Zak Crawley, Ben Duckett, Ben Foakes (wk), Jack Leach, Ollie Pope, Ollie Robinson, Joe Root, Mark Wood, Rehan Ahmed