Key opened the batting against the County championship favourites Lancashire at Tunbridge Wells in Kent and cruised to his sixth century of the season.
Remarkably it was also his fifth hundred in his last seven innings.
Key missed out on the mythical cricketing figure of 1,000 runs before the end of May by just two days.
He reached his hundred off 186 balls and, never the most athletic of figures, he eschewed the necessity of running by hitting 18 fours and a six.
He was eventually dismissed for 180 after more than six hours at the crease. Kent finished the day on 416 for four.
Key narrowly missed out on improving upon his career best of 199 against Surrey at The Oval last week. In that match he batted for more than 12 hours.
The right-hander, who turned 25 last month, has played eight Tests since making his debut against India at Trent Bridge in 2002.
The burly opener impressed the world champions Australia with his temperament during the subsequent Ashes tour.
But he was dropped almost exactly a year ago after the Test against Zimbabwe at Chester-le-Street.
He averaged just 18.76 with a highest score of 52, and was guilty of getting out to soft dismissals several times when well set.
But following Nasser Hussain's retirement after the first Test against New Zealand at Lords in May, Key's name has returned to the selectorial frame.
Hussain's obvious replacement is Andrew Strauss, who proved to be a brilliant stand-in for Michael Vaughan when the England captain had to miss the Lord's Test through injury, scoring 195 runs and winning the Man of the Match award.
But Hussain mentioned Key's name when he said that he wanted to retire in order to give younger players a chance to shine.
Another former England batsman Ed Smith also scored 116 as the red rose County were put to the sword on the first day of the First Division match.
Smith also received an England call-up last season after a similarly rich vein of form but like Key he failed to capitalise.
Key's opening partner and captain David Fulton made 43. Like Key and Smith before him, Fulton enjoyed an annus mirabilis in 2002 but failed to gain international recognition.