On a day when England captain Michael Vaughan was ruled out of the Ashes series after knee surgery, A team skipper Key's 128 not out was the centrepiece of the hosts' 268 for three on the opening day of this four-day fixture - Pakistan's final first-class match before next week's first Test at Lord's.
Key, 27, who had shoulder surgery during the off-season, played the last of his 15 Tests against South Africa at Centurion in January 2005.
He is still restricted in his ability to throw, so is unlikely to be in England's first Test squad due to be announced on Sunday.
But he did his prospects of a recall no harm at all, making the most of a reprieve after being dropped on 30, by batting all day on his Kent home ground to score his maiden first-class century of the season.
At stumps he had faced 250 balls with two sixes and 16 fours, while Ravi Bopara was unbeaten on seven.
"It's been a slow start for me in four-day cricket but it was a pretty flat pitch, one of those when if you got in it was going to be pretty hard to get out," Key told reporters after the close.
"What they (Pakistan) did brilliantly was to get the ball to reverse-swing, they showed some class with the way they got the ball to do stuff on a wicket where there was no conventional swing and no seam movement."
Turning to his Ashes prospects, Key added: "I understand it's going to be tough. You've always got that in the back of your mind.
"I took the captaincy at Kent to help me get back in the England side. There are still quite a few people in and around the team as well.
"My name will be amongst them somewhere, it just depends on where, but with a good end to the summer you never know."
Meanwhile Pakistan coach Bob Woolmer, a former Kent and England batsman, said: "I thought our players bowled well on a good pitch.
"It was good to get Mohammad Asif going again as he hasn't bowled for a few weeks," added Woolmer, whose team beat Leicestershire by eight wickets in a three-day match concluded Monday
England A lost Owais Shah (23) late in the day when he pushed forward defensively to leg-spinner Danish Kaneria and was caught by Imran Farhat at first slip.
Key, together with Ian Bell (74), put on 160 for the second-wicket in 45 overs after winning the toss.
Pakistan had a scare before lunch when Kamran Akmal, the only wicket-keeper in their squad, sustained an injury to his left index finger.
"It's just slightly bruised," said Woolmer, upbeat about Akmal's prospects of appearing at Lord's a week on Thursday. "He's got full movement, there's no break.
"Zulqarnain Haider, who has kept for the A team, is on standby and he's training every day."
Key, dropped by Akmal off Mohammad Asif before lunch, and Bell, unbeaten on 36, scored briskly.
Bell's fifty came off 112 balls with seven fours, while Key's half-century took 95 balls.
Key then drove Kaneria back over the bowler's head for six.
However, Pakistan broke the partnership when Bell, aiming to force through the offside, was caught behind by stand-in keeper Faisal Iqbal off a rising Umar Gul delivery, leaving England A 164 for two in the 49th over.
But Key kept going and a single off seamer Abdul Razzaq took him to a 191-ball century with two sixes and 14 fours.
Meanwhile pace bowler Rana Naved, struggling with a groin injury, came on as a substitute fielder.
Earlier Pakistan had opener Alastair Cook, set to be one of their Lord's opponents, caught by a diving Akmal for three after the left-hander pushed forward to Mohammad Sami.
England A 1st Innings:
Fall of wickets: 1-4, 2-164, 3-238
To bat: J Dalrymple, A Loudon, C Read, T Bresnan, M Hoggard, S Broad
Pakistan: Imran Farhat, Salman Butt, Faisal Iqbal, Mohammad Yousuf, Inzamam-ul-Haq (capt), Kamran Akmal (wkt), Abdul Razzaq, Mohammad Sami, Umar Gul, Mohammad Asif, Danish Kaneria
Toss: England 'A'
Umpires: Richard Illingworth (ENG), Darrell Hair (AUS)